I almost managed to figure it out how to preset that virtual button. Basically it's...yes...with your thumb but make sure you also secure the right side of the watch with your index or middle finger (whatever floats your boat). Also, your thumb must be placed a bit underneath that carved bar on the long side of your finger and not pointing it downwards (not sure how to properly explain it, but imagine you would grab it with a claw. That "claw" goes underneath the item to grab it). Also, it's a "pressure" button, so keeping it fixed with your index will allow you to actually simulate a PRESS action (don't just touch it lightly like the screen).
At least that worked for me after a few hours of pain.
