Required Fields - Project Association Area (when converting to a project)

I will dare to say this isn't possible by default.

The workaround that comes to my mind - use "native fields" in your custom form that would map to those fields and mark your custom/native fields as required while hiding the actual/native fields from the view. That way users should be forced to fill those in.
I admin I haven't tested it.

I had an idea for this: https://experienceleaguecommunities.adobe.com/t5/workfront-ideas/idea-ability-to-make-native-workfront-fields-required/idi-p/565850 and they have since marked it as "delivered". I haven't tested the functionality they propose solves this solution yet but here is a link to the documentation: 

https://experienceleague.adobe.com/en/docs/workfront/using/administration-and-setup/customize/custom-forms/form-designer/design-a-form/design-a-form#add-workfront-native-fields

"I'm glad to let you know that we now have a new functionality that allows you to add a Native field to a custom form, and when the native field is added to a custom form, you can make it required as well. 

To achieve the requested feature, you could hide the Project's Portfolio and Program fields from the overview section using Layout Templates and then add Portfolio and Program fields to the custom form and make those required."