I have this .desktop file.
[Desktop Entry]
Name=Tagger
Comment=Read and create QR code and barcode tags
Exec=/usr/share/tagger-app/tagger %U
Icon=/usr/share/tagger-app/graphics/tagger.svg
Terminal=false
Type=Application
X-Ubuntu-Touch=true
MimeType=x-scheme-handler/yada
I know it's being read because the icon appears.
I visit the link yada:// in the standard browser app and it says "network error" and does not attempt to open the application.