How to require the Suppliers to attach a copy of the invoice in Supplier Portal ?

Summary:

 How to require the Suppliers to attach a copy of the invoice in Supplier Portal ?

Content (required):

Hello

I was wondering if you can share with me how to make the attachedment field mandatory when the suppliers capture their invoices in Supplier Portal

Please Advise

Version (include the version you are using, if applicable):

R13

Code Snippet (add any code snippets that support your topic, if applicable):