How do i insert data on database based on id but showing the name on combobox?

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C#

I'm so confuded right now that i can't think a way to solve this thing...

Take this for example:

I have 2 columns, one with ID_empresa and one with Name_empresa


I want my combobox to show the name_empresa and insert id_empresa on another table, that has the column for id_empresa.

I would apreciate any help, this is giving me a pain in the ass! :/

My code:

namespace Portaria
{
    public partial class DarEntrada : Form
    {
        MySqlConnection con = new MySqlConnection(@"server=localhost;user id=root;password=12345;persistsecurityinfo=True;database=portaria;allowuservariables=True");

        MySqlCommand cmd = new MySqlCommand();
        MySqlDataReader dr;
        public DarEntrada()
        {
            InitializeComponent();
        }
        
        private void label1_Click(object sender, EventArgs e)
        {

        }

        private void DarEntrada_Load(object sender, EventArgs e)
        {
            // TODO: This line of code loads data into the 'portariaDataSet.empresas' table. You can move, or remove it, as needed.
            this.empresasTableAdapter.Fill(this.portariaDataSet.empresas);

            dateTimePicker1.Enabled = false;
            txtmatpers.Visible = false;
            txtsector.Enabled = false;
            button1.Click += new EventHandler(button1_Click);
            FillDropDownList();
        }

        public void FillDropDownList()
        {
            string SQL = "SELECT id_empresa, nome_empresa FROM empresas ORDER BY nome_empresa";

            DataTable dt = new DataTable();

            // Set the connection string in the Solutions Explorer/Properties/Settings object (double-click)
            using (var cn = new MySqlConnection("server=localhost;user id=root;password=12345;persistsecurityinfo=True;database=portaria;allowuservariables=True"))
            {
                using (var cmd = new MySqlCommand(SQL, cn))
                {
                    cn.Open();

                    try
                    {
                        dt.Load(cmd.ExecuteReader());
                    }
                    catch (MySqlException e)
                    {
                        // Do some logging or something. 
                        MessageBox.Show("There was an error accessing your data. DETAIL: " + e.ToString());
                    }
                }
            }

            // UPDATED - The .ValueMember and .DisplayMember properties 
            // refer to the string name of the field (oops!):
            comboBox1.DataSource = dt;
            comboBox1.ValueMember = "id_empresa";
            comboBox1.DisplayMember = "nome_empresa";
        }

        public void SaveComboBoxContent()
        {
            string SQL = "INSERT INTO entradas (id_empresa) VALUES (@id_empresa)";

            using (var cn = new MySqlConnection("server=localhost;user id=root;password=12345;persistsecurityinfo=True;database=portaria;allowuservariables=True"))
            {
                using (var cmd = new MySqlCommand(SQL, cn))
                {
                    cmd.Parameters.AddWithValue("@id_empresa", comboBox1.SelectedValue);
                    cn.Open();

                    try
                    {
                        cmd.ExecuteNonQuery();
                    }
                    catch (MySqlException e)
                    {
                        // Do some logging or something. 
                        MessageBox.Show("There was an error accessing your data. DETAIL: " + e.ToString());
                    }
                }
            }
        }
        private void chkmatpers_CheckedChanged(object sender, EventArgs e)
        {
            
            
            try
            {
                if (chkmatpers.Checked == true)
                {
                    msktxtmat.Mask = null;   
                }else
                {
                    msktxtmat.Mask = "00-00-AA"; 
                }
            }catch(Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }

        private void chkvariasemp_CheckedChanged(object sender, EventArgs e)
        {

            try
            {
                if (chkvariasemp.Checked == true)
                {
                    
                }
                else
                {
                    
                }
            }
            catch(Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }

        private void chksector_CheckedChanged(object sender, EventArgs e)
        {
            txtsector.Enabled = false;
            try
            {
                if(chksector.Checked == true)
                {
                    txtsector.Enabled = true;
                }
                else
                {
                    txtsector.Enabled = false;
                }
            }
            catch(Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }

        private void chkhoraentrada_CheckedChanged(object sender, EventArgs e)
        {
            dateTimePicker1.Enabled = false;
            try
            {
                if(chkhoraentrada.Checked == true)
                {
                    dateTimePicker1.Enabled = true;
                }
                else
                {
                    dateTimePicker1.Enabled = false;
                }
            }
            catch(Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }

        private void button1_Click(object sender, EventArgs e)
        {
            SaveComboBoxContent();
        }

        private void button2_Click(object sender, EventArgs e)
        {
            this.Close();
        }
    }
}

EDIT:

I have done some research and with 1 problem solved, comes another one, here is a printscreen of the error...
[^]


My Sql Query:

string SQL = "INSERT INTO entradas (id_veiculo,id_empresa,nome_condutor,empresa_visitante,empresa_visitar,visitado,ncartao,data,hora,obs) VALUES (@msktxtmat,@txtempvis,@txtnomecondutor,@txtempvisitar,@txtpessoavisitar,@txtncartao,@data,@hora,@txtobs)";

            using (var cn = new MySqlConnection("server=localhost;user id=root;password=12345;persistsecurityinfo=True;database=portaria;allowuservariables=True"))
            {
                using (var cmd = new MySqlCommand(SQL, cn))
                {
                    cmd.Parameters.AddWithValue("@msktxtmat", msktxtmat.Text);
                    cmd.Parameters.AddWithValue("@txtnomecondutor", txtnomecondutor.Text);
                    cmd.Parameters.AddWithValue("@txtempvis",txtempvis.Text);
                    cmd.Parameters.AddWithValue("@txtobs", txtobs.Text);
                    cmd.Parameters.AddWithValue("@txtncartao",txtncartao.Text);
                    cmd.Parameters.AddWithValue("@txtpessoavisitar", txtpessoavisitar.Text);
                    cmd.Parameters.AddWithValue("@empvisitar", comboBox1.SelectedValue);
                    cmd.Parameters.AddWithValue("@data", DateTime.Now.ToString("yyyy-MM-dd"));
                    cmd.Parameters.AddWithValue("@hora", DateTime.Now.ToString("hh:mm:ss"));
                    cn.Open();

                    try
                    {
                        cmd.ExecuteNonQuery();
                    }
                    catch (MySqlException e)
                    {
                        // Do some logging or something. 
                        MessageBox.Show("There was an error accessing your data. DETAIL: " + e.ToString());
                    }
                }

What I have tried:

I've tryied an example found on the internet, but now it says that the id_veiculo(which is the license plate doesn't have a default value...