I don't know if i got you right but if you want to set the manager of one of the list items via a dropdown, you'd have to make the list view editable (right click on the list view → top of first tab → select “yes” for “editable”) after that you should be able to put in a reference selector in the manager field.
Before that make sure your domain model allows the correct reference / has the necessary association!
Let me know if this worked out for you!
BR
Jan