Making a Field Mandatory on a Form

I have been trying to make a field mandatory on a form in AX 2009, I have tried the two ways i know:

• Going to the datasource field and making it mandatory in the properties, which gives me the red wavy line in the box, but still allows me to click on and off the control and close the form without inputting any data.
• And using a ValidateWrite method on the DataSource of the table;
  •  if(!creditCardTable.Zipcode)

                        ret = checkfailed(strFmt("Zipcode must be filled in!"));

•  
  •  if (!CreditCardTable.Zipcode)

{

warning("Zipcode must be filled in!");

ret = false;

}

Do you know any other way of making the field mandatory when the client will have to input/choose a value for the control before closing the form window?