Turn on the camera to start recording.
Add the bot to your channel as an admin with permission to post. Telegram's bot API accepts uploads up to 50 MB. Recording uses MP4 / H.264 + AAC where the browser supports it, which is what Telegram plays inline and lets you scrub. Firefox can only record WebM — it will still send, but as a file. Anyone who opens this page can read the token, so keep the file private or move the upload to your own server.