If you want to use ISBoxer's round-robin feature with a key (or set of keys), then that key needs to be assigned in ISBoxer somewhere. Unfortunately, assigning the left mouse button (Mouse1) means that it's going to be "eaten" by ISBoxer, when Key Maps are enabled. So, to counter that, you need to create a custom setup where you're toggling a Mapped Key (or Key Map) that is only enabled when you're within a certain region on the screen.

However, if that sounds like a lot to set up, then the alternative to this is to create multiple healing regions using Targets that are assigned directly to Slots (or Characters or ATGs).