Interaction of fornt end on react and back end on python flask?

Question

Flask

Hello, I already broke Google (maybe I just googled it wrong). In general, I am creating my application on react and flask and there was a problem, I need to add a handler (flask) to the form (react).
So, how is this done in these two frameworks, maybe there will be some examples? I'm still new to this type(?) of website development, so please-shout-beg, mayday! may-day!