There are several ways to do this. One way is to use code-behind in the SqlDataSource1_Inserting event. In the event code you can set the SqlDataSource1.InsertCommand property string to whatever you need. Something like this might do it.
protected void SqlDataSource1_Inserting(object sender, ObjectDataSourceMethodEventArgs e)
{
TextBox myTextbox = (TextBox)DetailsView1.FindControl("TextBox1");
string MySQL = ="INSERT INTO PURPOSE_MASTER(SRNO, CERTPURPOSE, EX1, EX2) VALUES (0,'"
MySQL += myTextbox.Text;
MySQL += "', 0, '--')";
SqlDataSource1.InsertCommand = MySQL;
}