To enable subtitles by default, you will need to use &cc_load_policy=1 parameter
3. Linking to Videos with Captions
If you want to share a link to the video for viewers to see captions by default, you need to add this code at the end of the URL string:
&yt:cc=on
If there aren't any existing URL parameters, instead of appending &yt:cc=on you may need to use /?yt:cc=on
For example https://youtu.be/GUfCPMCMORM/?yt:cc=on