you can't right now :( it's inconvenient. best thing to do as of right now is to post in threads you're interested in so you can go back through your post history to find it.
oh, i might be wrong in that case-- ngl i didn't see that, I must have skimmed over it in the patch notes. disregard what i said, but i don't see a button or anything to do it, so if it is a feature... it seems to be very un-intuitive lol
If you're on desktop, it'll be to the right of the forum thread, and if you're on mobile it'll be below the thread.
Above the "Recent Topics" box, there should be two buttons. One says "View My Posts" (which will take you to an overview of all the posts you've made on the forums). The button you want will be just below that one, which says "Subscribe to Post". Click that, and you'll subscribe to whatever thread you're looking at! If you want to unsubscribe to a thread, you can find the unsubscribe button where the subscribe button was!
I hope that explanation makes sense, it's a suuper easy button to miss! If you still can't spot it, let me know. I can grab and embed some photos for you to help later when I'm not on mobile :3c