Assist is already set up in ISBoxer under keymaps -> general -> FTL Assist me. I don't think it comes with a hotkey so you will have to assign one. I use "G" personally.

Now to answer your question, Create a step in your IWT key, "do mapped key action", keymaps "General", and mapped key "FTL Assist me". Also assign a target like "Window: all without current".

That should do the trick if I understood your question right.