What are you trying to accomplish by showing a dropdown list? In the current domain model snippet, an instance of Entity1 can be referred to by multiple instances of Entity2. In a dropdown you can only select one item, so this is probably not what you want.

If you want to be able to select one instance of Entity2 in a dropdown list to set a reference on Entity1 then you probably want to change the association to

(Entity1) * ----> 1 (Entity2)

That way an instance of Entity1 can have a reference to one instance of Entity2. Using a reference selector you can then show a dropdown list with all instances of Entity2.

Edit

Normal dropdowns always work on an assocation, so I think what you could do is add an extra association, just for viewing purposes and put a data view over that association below it. However if it's going to be a big list (which you probably hope for if it's about bills) it might be more user friendly to use a list view instead, in which you can search. Then you could add a click action to open a page showing that object.

I always use

(Entity1) 1 <------- * (Entity 2)

(Entity1) * -------> 1 (Entity 2) dummy reference to achieve dropdown, on commit empty reference. Use xpath or constraint by to limit dropdown list