Field validation question

I am trying to add some field validation in the Order details form. Basically, I need to set some fields as mandatory. I have been able to add the asp:RequiredFieldValidator control, and while it does display the error message when the form is submitted, it does not really make the field mandatory, as it still submits and accepts the data.

How should I make fields mandatory (PO Number, for example)?

NetPoint Focus 5.96

SAP BO 2005A, SP01, PL19