At a guess, you need something like this:
string query = "select * from tblcustomer where " + myValueFromTheComboBox + "= @2";
SqlCommand cmd = new SqlCommand(query, con);
cmd.Parameters.AddWithValue("@2", txtvalue.Text);
You can't pass column names to SQL via a parameter, they are resolved before parameter substiution is performed.
But don't do this from a textbox as you code shows - that is very dangerous, as it leave you open to SQL Injection. A Combobox is relatively safe if you control exactly the content (but I'd probably still want to verify the content before concatenating it into the command string).