Create parent-child relationship when parent primary key is from composite key from sync

I have two tables, Patient sync (parent) and Labs sync table (child). 

The Primary Key of Patient sync table is a composite key Jane_Smith_janesmith@fake.com. This is created during the automatic syncing. 

The Patient Labs sync data has the first name, last name and email of the patient in it in 3 separate fields. I want to connect these two records automatically when they sync if those 3 fields match.

When I try to create a parent child relationship, QB does not give me the option to set a reference field. It goes straight to a lookup field. I I configure the relationship, this is what I get, but the Related Healthie Patient field doesn't populate with anything because it is not the formula field from the lab child table. 


Is there some restriction on composite fields that I don't know about? Any suggestions about how to get around this and link these tables automatically during the syncs?

------------------------------
Solace Sonder
------------------------------