I have a custom entity that is related to 2 other custom entities and Contact.
I want the Add existing button to be available only on one of the custom entities.
I can either make the button disappear for all entities or appear for all.

This is the rule I created to try and hide the add existing button. No combination I try seems to work.
I also tried creating a rule that will make it visible on the single entity and that doesn't work either.
I am trying to do this on premise with legacy forms. I have also tested the new forms and not working there either.
I know I can probably hide it with Java Script but I would like to use OOTB functionality.
Thanks