@Christoph
Custom signing keys is not currently a supported feature for trusted launch VMs with secure boot. At this time, you can only enable the feature with the method described here.
For product feedback and feature requests I will refer you to our feedback forum. This allows the community to add their voice and upvote popular ideas. The forums are monitored and responded to by our product teams.
https://feedback.azure.com
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Please don’t forget to "Accept the answer" and “up-vote” wherever the information provided helps you, this can be beneficial to other community members.