Use the following settings for HLS Push:
Ingestion server: https://a.upload.youtube.com/http_upload_hls?cid=crmp-9fcz-fdzz-9k72-fkrj©=0&file=test.m3u8
Segment filename: /http_upload_hls?cid=crmp-9fcz-fdzz-9k72-fkrj©=0&file=test%03d.ts
The bold text has to be replaced with your stream key available in YT studio.
More details can be found here: https://developers.google.com/youtube/v3/live/guides/hls-ingestion