Empty option caption cannot be filled with Microflow

Hello,  I have a dropdown list in a popup view. The dropdown list is filled from a previous list with items. When you press on the arrow the dropdown list is get the list of categories retrieved from a microflow as expected.  The challenge what I encounter is to fill the “Empty option caption” in a dropdown list from an attribute in object from a microflow. The idea is to lookup the value of the category based on the id of the “Vraag” 1.    How to trigger a microflow for the “Empty option caption” and put the result in this field? 2.     What other method can I use to achief the challenge ?        
1 answers

According to the Mendix douments you can write only text in the field of Empty option caption. As you know, it will be shown if nothing is chosen or if there is nothing to choose.