Github is not really a place to store user data if that’s what you’re thinking (Might be against their ToS), also there might be rate limits. If you plan to store user data externally via httpservice, you’ll need to setup a database somewhere that you can use, such as on a VPS, dedicated server, or some other service like AWS. Don’t rely on free services, or do hacky ways to store things (an example would be trello)
If you have no idea how to go about doing any of this, i’d recommend learning how to setup websites using nodejs, mysql, and nginx.
Another thing to note is hosting data may not be cheap. For a small game that has barely any players, it’d not be a problem, however if your game expanded to thousands, your storage could have problems, so be aware. Running databases can be challenging at first (especially if you have no experience).
As to answer your question, it depends on the website you’re saving to, as each website has different URL layouts, and different headers. most would likely use a POST request.