This is kinda convoluted and not very intuitive.
- Create a contact group: WATO → Contact Groups → New Contact group
- Create a User with the E-Mail-Address you want the notifications sent to: WATO → Users → New Users and add him to the contact group! The tab may be collapsed by default, so it’s easily overlooked. It’s the third panel from the top, right under “Security”
- Create a notification rule: WATO → Notifications → New Rule. Then add the newly created contact group. Expand the tab “Contact Selection” and check “The members of certain contact group”. Next, click “Add Element” and select the newly created contact group.
- Save and activate changes.