it was tricky for a while tbh, you basically have to input the u2 super early, like even earlier than other strings. Do f2 and then immediately slide to u2 before the f2 even comes out.
In most combos you can do a stand 1 before f2u2 which gives you a small window to do it properly as opposed to doing it after a run or a njP, but yeah you basically have to grind it up in training for a bit.
While you're there grind stanky leg's input as well.