If you have both objects in the microflow then change the object which “owns” the association. This can be identified by the dot of the association.
For example having this domain model
And in a microflow I want to create a CartItem for the Cart. The CartItem is the owning object of the association. When creating the object (change object in case that the object already exists) I can select the association member and set the reference to the object variable I want to reference.
Hope this answers your question.