Not to excuse these dark patterns from shitty products that can't get revenue without tricking people, but privacy.com lets you set up single-use credit cards with e.g. a $1 lifetime limit. This is my go-to for places like this if I really, really want to try it.
You might enjoy the binaural beats and ambient radio on https://ambiph.one as a free alternative (disclosure: it's my site). Not as science-y as brain.fm but it helps me a lot with focus