Once you’ve selected an ingest server, and added your stream key to it, you’ve got all you need from the API.
Now you simply use RTMP to send your stream to that ingest and you’re live.
If you need to change your title/game you’re playing then that can be done through the API https://dev.twitch.tv/docs/v5/reference/channels#update-channel but as for actually starting the stream once you’ve got an ingest server and your stream key you have everything you need.