If it's not something you have access to on Beyond, you have to manually construct it - you can't pull from any websites other than Beyond or anything, that I'm aware of, not with all the syntax done right and stuff.
So if you have something in a physical book, but want it on Beyond without paying the slightly exorbitant single-item fees, you're going to need to just write it up yourself. Which may mean learning a bit of syntax and so on. The easiest way to do this is to copy the most similar subclass/spell/monster/whatever, then just work from there. I suspect there's a good summary of all the syntax somewhere but I have yet to find it so you may want to make copies of a few things to see how they work. I managed to put together a Druid subclass which functions entirely correctly (the Sacrifice Druid from Odyssey of the Dragonlords) recently myself.
Please note they Beyond is totally clear that manually recreating stuff from books (even ones you don't own) is absolutely fine. You can absolutely do this, you just can't share outside campaigns you're linked to (i.e. you can't upload it to the central store thing they have).
