From https://issues.redhat.com/browse/SAT-3236:

It would be a user input similar to the Value Type search, requiring a Resource Type.
When such input is rendered in the invocation form, it creates a select2 (or similar) drop down field
allowing to search in all resources of a given type, e.g. ActivationKey.
The macro loading such input, e.g. input(‘registration_activation_key) would then return
the selected ActiveRecord instance of a selected ActivationKey, so the value would be stored as its id.