Explaining Tim's answer further, under selectable objects you can write a microflow/nanoflow which is responsible for controlling the objects list shown in drop-down. Inside that microflow, you can get the entire list and then create a new list as 'resultlist', iterate over the entire list and add distinct objects to resultlist. Hope this helps.
What will be the if condition which will be true to add to list. I have tried this but still no output. Can you check the mistake and let me know.
Use the tab "Selectable objects":