Head on over to your YouTube live Events page (https://www.youtube.com/my_live_events).
Create a new live event that is unlisted (or private) and of Custom type.
Under Basic ingestion choose 1500 Kbps - 4000 kbps (720p).
Check the Enable 60fps box.
Note your Stream Name and Primary Server URL.
Run the following in a new terminal window (replacing <m3u8_5000kbps_url> and <youtube_rtmp_address>):
ffmpeg -re -i <m3u8_5000kbps_url> -c:v copy -c:a aac -ar 44100 -ab 128k -ac 2 -strict -2 -flags +global_header -bsf:a aac_adtstoasc -bufsize 3000k -f flv "<youtube_rtmp_address>"
Example:
ffmpeg -re -i protocol://domain.tld/stream.m3u8 -c:v copy -c:a aac -ar 44100 -ab 128k -ac 2 -strict -2 -flags +global_header -bsf:a aac_adtstoasc -bufsize 3000k -f mp4 "rtmp://a.rtmp.youtube.com/live2/<STREAMNAME/KEY>"
Head back to your YouTube Live Event.
Click on the Live Control Room tab up top.
Click the Preview button.
The stream should take a moment to preview, then you can click the Start button.