I had same problem earlier no matter what I tried it wouldn’t load in steamvr at all (not even steam theater mode).
I had to get VorpX to install a special hook helper and then next time I started it I finally saw it in steamvr. I don’t know how you’re really supposed to do it so maybe someone with more experience can chime in but when the game was starting and it popped the little “hooking forzahorizon5.exe” window I dragged that to my second monitor and at some point it popped up an advanced dialog where I could select to install the hook helper. It took a long time for that advanced dialog to pop up and I can’t remember if it was after I exited the game or not
I have not tried all of them, So far I have tried cyberpunk, f1 2021. I ended up using reshade to download the superdepth 3d and am using bigscreen with sbs. I still need to tinker but i wish i could get VorpX to hook, so much easier and better when it works.