My sales team regularly creates custom object records and associates them with deals, companies, and contacts. It would save a lot of time for them if from the custom object record creation form, deals and companies were automatically associated with the new record.
This seems possible, becuase already a deal is associated by default when you start the custom object record creation process from a deal record right hand side bar. The Company and Contact fields should populate by default with whatever records are already associated with the deal.
Example:
This ‘Job Opening’ creation form automatically associated a deal because I added a new Job Opening from that Deal record. The deal has associated Companies and Contacts, and they should also be included in these field by default.

