Hi @advelita
There’s an old tutorial with the similar topic, which you can see here: https://www.bobz.co/dynamically-populate-select-fields-choice-in-advanced-custom-fields/. Unfortunately, I’m not sure if it’s still working or not.
If you need the relationship, not just assigning a value to another one, then I suggest you create custom post types instead. That way you can use the relationship field or post object field to create the relation between the two object.
I hope this makes sense 🙂
