Custom Unique IDs not supported by Forms

Dearest experts, how can I find my way around to associate a Contact and other object (ie Company) in Contact Form when our customer is strongly BTOB focused and uses custom Company IDs to handle incoming requests. We do not want each request to create duplicates and yet, the company has to be associated to the Contact (if new/created) exactly like the solution would operate for imports of associated objects. I assume this has similar impacts on those who wish to associate ticketing request to Companies or Orders etc…  Note : For Companies, Domain isn’t a sufficient ID at all. This is a BIG struggle for me, how do you handle that?