Learn the hanging pike
The curl of the abdomen in any leg raise is the meat of the movement. Without it, you’re always going to be using the hip flexors as a primary mover.
Not using any momentum, keeping your head between your arms and your chin towards your chest, and not engaging the lats or leaning backwards is the proper way to a leg raise. Having a partner press the small of your back (or using stall bars) as pictured in the article is a good method to isolate correctly.
If you don’t have the strength to do the movement in this manner, hanging knee-ups / tucks, concentrating on the curl of the abdomen (again), is a good substitute. There’s also a progression in the article I’ve linked to.