Option 1: (simple) Format your field as a pop-up menu instead of a drop down list.
Option 2: (not as simple) If you really want a drop down list, you do what you want with a little slight of hand. Format your field as a drop down list that uses the arrow feature. Shrink the width of your field so that only the arrow is visible. Place your related name field adjacent to the arrow so that it looks like one field. Now assign a script trigger that uses Go To Field to automatically enter your drop down formatted field each time the related field is entered--that way your drop down should appear whether the user clicks on the arrow or when the user enters the related name field.