Adding a +1, +2 or +3 to a custom monk item for stunning strike save DC?

My dm gave me a homebrewed item and it was so I could use Stunning Strike more often with a chance of it working and its a +1 to the save DC at the moment but when trying to make the item in the homebrew item there is no selection for any of the monk features for the bonus of a magic item. There is a bonus for nearly all other classes only other class that doesn't seem to have a bonus for homebrew magic items is fighter. I don't know if these were just missed on update or thought they weren't needed but it would be a lot of help if we could get them added in please.

There is, unfortunately, no way to do this with the current homebrew system. In fact, the character builder doesn't even properly support official items that do this kind of thing (e.g. Dragonhide Belt from Fizban's Treasury of Dragons) so this will probably have to wait for the big character builder overhaul that's allegedly coming.