First, people on dual-boxing.com are going to flip out because you want to both "assist" and "interact with target" without "pressing multiple keys". IWT can't be put inside a WoW Macro, so basically the great Gods of dual-boxing.com will smite you for not sticking to "one key one action". For the record, I believe this is absolutely harmless and "one key one action" as interpreted by various dual-boxing.com members is nowhere near the ToS.

Did you know? World of Warcraft mitigates button mashing by using Global Cooldowns (GCD)! It's impossible to use two GCD Actions without a delay, which is impossible to create using ISBoxer. Any delays are purely based on your hands touching your input devices and affecting your available controls (which may not all be spring-loaded buttons).

All that said, you can make it Assist on button DOWN, and IWT on button UP, and then we're still talking about 2 operations for you to do 2 things in WoW. Or both of these things can happen in the same step since neither is affected by GCD.

So, make a Mapped Key with one or two steps, with 2 Actions:
1. a Do Mapped Key Action. Target is "Window: Current", Key Map is "General" and Mapped Key is "FTL Assist Me"
2. A Keystroke Action. Target is "Window: All w/ Current", Key Combination is F1 (because this is what you configured in WoW for IWT)

That's it. They can go in Step 1 in that order, or you can put one in Step 1 and one in Step 2. If you do 2 steps, you can set the "Execute a Step when the Hotkey is _____" to "pressed OR released" to do one Step when you press the button and one when you let go of the button.