C# add data to sql table

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C#

SQL

Hey,

I have a form that user can add a new client and it inserts data into database by a button click.

this is my code that worked perfectly

private void button1_Click(object sender, EventArgs e)
{

    string sqlText = "SELECT MAX(customerName) FROM Customer";

    SqlConnection sqlConn = new SqlConnection(@"Data Source=Admin-PC\SQLEXPRESS;Initial Catalog=Couriers;Integrated Security=True");

    SqlCommand command = new SqlCommand(sqlText, sqlConn);


    SqlDataAdapter da = new SqlDataAdapter();
    da.InsertCommand = new SqlCommand("INSERT INTO Customer VALUES(@customerName,@CustomerAddress,@CustomerContact,@VatNo)", sqlConn);
    da.InsertCommand.Parameters.Add("@customerName", SqlDbType.VarChar).Value = textBox4.Text;
    da.InsertCommand.Parameters.Add("@CustomerAddress", SqlDbType.VarChar).Value = textBox2.Text;
    da.InsertCommand.Parameters.Add("@CustomerContact", SqlDbType.Int).Value = textBox3.Text;
    da.InsertCommand.Parameters.Add("@VatNo", SqlDbType.Int).Value = textBox5.Text;
    
    sqlConn.Open();
    da.InsertCommand.ExecuteNonQuery();
    sqlConn.Close();
    MessageBox.Show("Client Successfully added");
    this.Close();

}

Now i added CustomerId into my SQL table and having problems with this code.
The customerId must be generated automatically dont want user to input id, but now the code is wrong