It’s currently not possible to associate contacts to other objects (e.g. company) by using custom unique identifiers of a company.
Example form A:
email (contact property)
domain (company property)
--> The contact will be associated to the company even if the company is already in the CRM. This is good.
Example form B:
email (contact property)
Custom ID (company property)
--> The contact will be associated to the company but only if the company doesn’t already exist in the CRM. This is bad.
Also it would be great if there would be a possiblity to use record ID in forms. E.g. for a support form where contacts would be able to update data for an already existing ticket (but only if they know the record ID of course).