Can't find a User in a list located within a seperate Thing

I am using Bubble as a back end for a Dropsource app. I have a list of Users located within a Thing called a NetworkGroup. I am trying to retrieve those specific users to be displayed in my app. The issue is that if I set up an element in Dropsource to show that list, it will display the Unique ID field of each user and nothing more, which makes sense.
Since I would like it to display the Username field, I decided to create an API endpoint within Bubble to get the Users within the NetworkGroup list. The problem is that when setting up a constraint within a Bubble search, if you click on the Unique ID field, it only allows you to use the “=” function. Other fields allow you to use “is in” which is what I would like to be able to use.
*Is there any way to make changes to users stored in a list in another Thing?