Make the AGM entry field mandatory on a Smart Form

We are trying to figure out haw to make the Agreement field on a request form Mandatory.

We have several Smart Forms/Requests forms that are to be used for requests for goods or services that are on CPA (Agreements)

The requesters enter the AGM # into the agreement field so it defaults to the appropriate Supplier. They do not enter the supplier.

On occasion they do not enter the AGM # but the req can still be submitted. We want to requisition to stop if a AGM # is not entered.

the smart forms section of Catalogs does not allow you to do this.