Thanks for the replies everyone! Mendix Studio doesn't have a Reference selector, but in the post Radhika mentioned someone said you can create a new attribute with a different name (eg. title1) after which the attribute list does show all the original attributes. This works.
I think this post might be helpful → https://forum.mendix.com/link/questions/112868
Also, I think Studio only has the option of Dropdown (last time I checked) and Studio pro has a different convention of calling it as a reference selector.
Hey Stijn, thanks for the quick reply!
Would this mean that I am reading the Academy wrong or what's up. Cause the Academy specifically states to use a drop-down:
A dropdown can only be used on attributes of type ENumeration. (https://docs.mendix.com/refguide/drop-down).
I you want to select an attribute over an association, you should use the reference selector.