The longer video, with further exercises, is on my YouTube channel.
These are some abs exercises to try on the pull up bar. If you also want to work your forearms and back, perform them at the top of a pull up.
01 Leg raises
Keep your legs as straight as possible and raise your legs to 90 degrees.
02 Toes to bar
These are like the standard leg raise, but instead of stopping at 90 degrees, raise your legs all the way to the bar. Keep your upper body as still as possible and avoid leaning back.
03 L seat to straddle
I'm sure we used to call these something else, but I can remember the name! Start in a L seat and go as wide as you can with your legs, close back to an L seat and repeat.
Slightly easier than the exercise above. Use small controlled movements to move your legs in an X shape.
These are a good exercise if you're struggling with clean leg raises, toes to bar or L seats. Raise your legs, individually and one after the other, to 90 degrees.