How to add a validation when add or update data in "Sales Line Discount" ?

Hi expert ,

   I have added a field( 'field1' ) in "Sales Line Discount"  . Navision have already provide a validation with few fields . Now I want to include my field ('field1') in validation when my field is non empty. How I can add this type of  validation in "Sales Line Discount" ? Please give me suggestion .

Thanks

Surajit