Unlike how you can type into the box to add a new medical condition, you can't create new dietary requirements this way. Typing into the box when assigning a dietary requirement to a student or staff member simply narrows down the search list.
You can see a full list here: Arbor's list of available Dietary Requirements
If the requirement isn't on the list, select Other, and add a note to go alongside explaining the dietary requirement. You can see the note when reporting on dietary requirements from the School > Meals > Dietary Requirements page.