Orgasms are a physical thing, with physical prerequisites. This is a good thing! It's your body, you can train it!

It sounds possible that the way you tense your legs when you masturbate engages your pelvic floor and lower abdominal muscles, which, when combined with clitoral stimulation, eventually brings you to orgasm. Everyone's anatomy is different, but there are other ways f engaging those muscles (and making them stronger!). You can do sets of kegels (think picking up a grape and then crushing it). try doing kegels while playing with your clit (breath out on the contraction, and give it a little extra squeeze, like you're trying to squeeze the last drop out).

But in general...learn about your body. Like there are books. "Come As you Are" but emily Nagoski is a good start, but beyond that, muscular development is a thing. Lots of women have underdeveloped glutes, and so their hip flexors, psoas, and quads pull everything out of alignment. Or some stuff is tight and makes it hard to release (which is also necessary for orgasm).

The overall point I'm trying to make is that while everyone is different, the female body isn't some indecipherable rubix cube. You can learn about yourself and strengthen or change whatever you want to. It's empowering. :)