@ThinhDinh correct, I’m going for a form-in-form situation 
That unique ID for plans_working could presumably be row ID? I just tried changing the custom action for the button on plans_working so that it sets temp plan ID for sections_working to the row ID of plans_working, but the button didn’t appear when using that method.
Speaking of custom forms, I tried to use Darren’s custom form concept but I can’t seem to copy it - is there another tutorial somewhere on custom forms?