So after looking at some threads I wanted to confirm if this feature just doesn't work.

I am trying to have a homebrew race start with 100 Hitpoints. I figured going to Bonus - Hit Points and then Fixed Value - 100 would do something. But it doesn't. I have tried different combos of such things and nothing seems to change the races hitpoints.

To extend my question is other things like using alternative ability scores to add into health besides constitution, or say having double con modifier be added just not possible with the homebrew tools on dndbeyond?

There are a couple of features that use bonus hit piunts per level, but I don't know if anything official uses a single hp bonus. It may be a leftover/extra modifier that doesn't have any function (a few of those exist).

And you didn't ask for feedback, but starting with 100 extra HP is overpowered. Just my 2cp.

No worries about it being overpowered, it is the type of games we enjoy and I never have had an issue challenging my table.   Sometimes a story isn't about struggling to be a hero, but also what it is like to be a God among mortals, and what those power fantasies are like.

Though regardless of that, yeah I wanted to see if it was a dead feature, a bug, or if I was just misunderstanding something. It was annoying having to edit hp all the time, I was hoping I could just adjust it in homebrew so it all happen behind the scenes.  Less work on my end if I could also just switch it so instead of con, say  wisdom or another ability score is added to health.

Anyways thanks for the reply!

There is no modifier for this. You can include the feature and just put in a note that the player will need to edit their health manually. The player can then edit their char, use Manual HP and just type in 100 plus class/rolled health. 

Yeah, that is what we have been doing. Just adding it up as if it was rolled health.