I want to give my player a magic item that increases the charm and frighten save by two but can't figure out how to add that in on the creation page.

I would probably add the modifier:

Modifier: Bonus
Modifier Subtype: Saving Throws
Details: +2 on saving throws against charmed and frightened (or words to that effect)

That way a little note will be added into the saving throws window - but you won't be able to make the dice roller take it into account. It's just a note for the player so they maybe might possibly unlikely remember to add it themselves.

thank you I appreciate it