I have a Twitch and want to use RTMP for live streaming, but I can't find its streaming code. How can I find it?

This is the official help document, you can refer to it.
https://dev.twitch.tv/docs/video-broadcast/

Specifically, the push stream address for Twitch is roughly as follows.

Server address:
rtmp://sfo.contribute.live-video.net/app/

You can access the key information in https://dashboard.twitch.tv Find it in the "Settings ->stream" menu.