You cant currently, sorry.
Ok, that’s not totally true, You can in XTP Web player as long as the subtitle is in vtt format. You can convert srt files to vtt on lots of websites.
You need to name the vtt file the same as the video and funscript for XTP to see it. It should auto load it in the web player. You can also download it from the web player if the web browser supports it. Spoiler alert, no browser on the quest 3 will download it last I tried. I dont know why.
Still should work in the XTP web player, just not external players like heresphere.