Supplier Site Field Required

Summary

Is there a way to make the Supplier Site field truly required?

Content

I have been working in page composer to make the supplier site field required for negotiation on the supplier train stop. The only field that is available is 'show as required'. This only puts an asterisk on the supplier site field but still lets the sourcing agent publish the negotiation without specifying supplier sites. Is there a way to make this field truly required?