UserDomain can't be added to app settings via Arm or ADO

[rowbot-99]

Please provide us with the following information:

This issue is for a: (mark with an x)

- [x ] bug report -> please search issues before submitting
- [ ] feature request
- [ ] documentation issue or request
- [ ] regression (a behavior that used to work and stopped in a new release)

Minimal steps to reproduce

deploy to azure using arm template with userdomain defined as a parameter

Any log messages given by the failure

The data is not valid (Invalid or not allowed environment key name: UserDomain

Expected/desired behavior

add userdomain as an app setting to web app

OS and Version?

Windows 7, 8 or 10. Linux (which distribution). macOS (Yosemite? El Capitan? Sierra?)

Versions

Mention any other details that might be useful

I tried deploying using custom template and azure devops, both failed with the same error. removing the param and app setting from the template deploys the app successfully.

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