I'm trying to work on some homebrew monsters and I was going to add to the monster description that it utilizes the primal companion feature from Tasha's Cauldron of Everything, but I'm not finding any online reference to achieve this outside of using external links which I'd rather not do. Is this possible to do since it's an official source or is there no functionality for this with the expanded sources?

There are no tooltips for class features, optional or otherwise.

What about giving a homebrew monster disadvantage on a skill such as stealth? The monster has bioluminescence so I'd like to give it disadvantage on stealth checks.

You could add it to the statblock as a rollable, but the DM would still have to long-hold the rollable to bring up the popup that will allow for them to manually select advantage.you can follow the link in my sigline to instructions on how to set up rollables.

Alright, thanks for the help :)