How to create an signed APK with own certificate?

Hello, Sven!

Unfortunately, we did not implement UI for APK publishing yet, here is the corresponding feature request: https://youtrack.jetbrains.com/issue/RIDER-8485

However, you should be able to publish the app via command line. These articles might be helpful:
https://developer.xamarin.com/guides/android/deployment,_testing,_and_metrics/publishing/signing-the-app-package/manually-signing-the-apk/
https://forums.xamarin.com/discussion/105762/create-a-signed-apk-ipa-for-qa-testing-using-command-line