I'm new to Helix, but I figured this out. I have one of the Mission pedals WITHOUT a toe switch so the question is, "How do you get the pedal to go on/off like it has a toe switch?"
The answer is to either (a) use the bypass hardware button or (b)...
1. select the wah block,
2. hit the hamburger button,
3. select "controller assign".
4. The leftmost button/knob controls the "parameter" you wish to change. First select "position"
5. Then the second-to-leftmost knob labeled "controller" will allow you to select exp pedal 1/2/3 (among other things).
So far so good. I think this is how far most people get.
6. Now use the leftmost knob again to select "mix".
7. At this point, the second-to-leftmost knob labeled "controller" will allow you to select a footswitch position for on/off functionality.
So toggle off and on with the switch, then use the expression pedal to wah. Not the best solution bc the on/off doesn't go from preset to preset. You'll have to set it for all presets that you use. But it's something.
Note, I'm on 2.54 firmware. Not sure if it applies to earlier versions.