Before anything, make sure to root out any incentives for bad posture from your environment. Just like how tall people sometimes stoop to hear/see the people they're talking to, you may be hunching as a "solution" to some problem in your environment. If you don't fix those problems, any posture adjustments will be short lived.

Some low-hanging fruit there: Make sure your screen is at eye level (when sitting up straight.) Make sure the font sizes on your computer are big enough that you can read without moving your face closer to the screen.

Folding your legs is also going to lead you to hunch if you're not flexible enough to sit up straight while doing it. (I'd wager that most western adults aren't.) Make sure your chair is at a height where you can have your feet comfortably on the ground. If the height doesn't work for your desk, get something to put your feet on. (My wife uses a laundry basket.)

In case of a dire need for intervention, strap your feet into a snowboard while working. (Half joking, but with my own ADHD, I feel like I'd need to physically restrain myself to break a deeply ingrained habit like that.)