First things first, turn off the Rich Text Editor. It’s in your profile options. It might look simpler without it, but you actually get a lot more freedom this way.
You’ll be able to enter these characters with what’re known as HTML entities (e.g. ä for ä). Here is the list of the entities you would need:
Ä ä ← Ä ä
Ö ö ← Ö ö
Ü ü ← Ü ü
ß ← ß
Edit: This will not work in the story title field, though. Looks like you have already, but you will have to make do with ae, oe, ue, and ss there.