How to make custom x-scheme-handlers work in AppImages?

Question

I'm trying to make a custom x-scheme-handler MimeType work in an AppImage I build. It should enable me to open said AppImage if I click a link in the form of my-handler://login-callback.

I already tried the following things:

1. Add the MimeType to the .desktop file in the AppDir directory -> Hint: adding the type to a local .desktop file and putting this into /usr/share/applications usually works
2. Add an appimaged.yml file with the following content into AppDir:

name: MyName
description: MyDesc
mimetypes:
  - x-scheme-handler/my-handler
desktop: MyName.desktop

I bundled the AppImage with appimagetool.
Does anyone know how to solve this or is it just not possible?