Hi Corina
Please correct me if I am wrong, but I think you are asking whether you can use a microflow or a nanoflow to fill a value in an association. If that is the case, rest assured, you can.
First a quick refresher: in order to use the reference set selector you will need an association between two entities in a many-to-many fashion. The reference set selector will be used show you the associated object(s) of the other entity to this entity.
You will need a couple of things before you continue
By the end it should look something like this (at the easiest):
Good luck!