It looks like you are searching for the constrained by function. See the documentation here: https://docs.mendix.com/refguide7/reference-selector#constrained-by
It’s for Mx7 but works the same in Mx8.
Or am I missing what you are trying to achieve?
Indeed pictures are a bit broken at the moment, they are working on a fix. Some assumptions here. I assume you have two references from this object: one for the pickup point and one for the destination? Indeed a on change microflow should be the fix. You pass the current object as parameter so you can retrieve the associated pickup point and destination. You can then do the stuff you want to do with either pickup point or destination.
I have an application with Destinations and Pickup Locations, where a Pickup Location could be used by multiple Destinations.
The reference selector should be the names of destinations, but once the form is saved I want to save the selection into the Destination Name value of the Pickup Location.