is it possible to make field mandatory?

Hi All,

While creating purchase requisition we are entering customer PO number in a filled (alias : URAT ), but some time user forget to enter value in this field. Once we created purchase order from requisition we are not able to edit this filed (Since it is disabled for editing). Is it possible to make this field as mandatory ? so user will get warning or error massage.

How do i do this?  Also suggest me any work around or solution if have?

Thanks,

Rajesh