Pop up message/DFF default value requirement in redwood RSSP page

Hi

We have enabled the Responsive Self Service (Redwood) page for requisitions. We have some 3rd party integration for risk assessment, which is pre-requisite for raising Non-Cataloge requisition. To capture that in Fusion Requisition page we have created a DFF field, where all users need to mention the document number which is created in risk analysis.
Now we want to show some pop up message to let user NOT to miss DFF value to be filled. only if it non cataloge requisition. Other smart form + Punchout requisition should get submitted without submitting.
To achieve it want to some of or either of points -