NOTE: there are a million stackoverflow answers explaining inserting into lookup fields that talk about __c fields. They are NOT duplicates of this question.
so I'm trying to insert a value into a lookup field using the rest api. If the field name looks like blah__c - it's easy - I just insert a {key, value} into blah__r. I do that all over the place.
but in this case my field is called PlanId. Trying to insert into PlanId__r just says no such field. How do I do it for a lookup field that does not end in __c?