*Write this code on the Button*

private void
button1_Click(object sender, EventArgs e)
        {
            SqlConnection con = new
SqlConnection();
            con.ConnectionString = "Data
Source=home-1117080bc2;Initial Catalog=Hotel_Booking;User
ID=sa;Password=sqlforarijit";
            con.Open();
            SqlCommand cmd = new
SqlCommand("select * from new_reservation where customer_id = @a",
con);
            cmd.Parameters.Add("@a",
SqlDbType.VarChar, 50);
           
cmd.Parameters["@a"].Value = comboBox1.Text;
            SqlDataAdapter ada = new
SqlDataAdapter(cmd);
            SqlCommandBuilder build = new
SqlCommandBuilder(ada);
            DataSet ds = new DataSet();
            ada.Fill(ds,
"new_reservation");
            dataGridView1.DataSource =
ds.Tables[0];
        }