how to fill combobox in c# windows application from database

Question

cmd = new SqlCommand("insert into Additem(Product_Name,Description,Qty,Unit_Price,Tax) values(@Product_Name,@Description,@Qty,@Unit_Price,@Tax)", con);

con.Open();

cmd.Parameters.AddWithValue("@Product_Name", producttxt.Text);

cmd.Parameters.AddWithValue("@Description", destxt.Text);

cmd.Parameters.AddWithValue("@Qty", qtytxt.Text);

cmd.Parameters.AddWithValue("@Unit_Price", unitxt.Text);

cmd.Parameters.AddWithValue("@Tax", comboBoxtax);

//cmd.Parameters.AddWithValue("@Tax",comboBoxtax.GetItemText(comboBoxtax.SelectedItem));

cmd.ExecuteNonQuery();

con.Close();

MessageBox.Show("Record Inserted Successfully");

DisplayData();

ClearData();