Hello @Jan Vávra ,
Thanks for reaching out.
Unfortunately, Azure AD Application Proxy does not yet support X-Forwarded-Host. However, you may upvote the following feature request so that may get included in the feature release.
https://feedback.azure.com/d365community/idea/7d830ecc-ba25-ec11-b6e6-000d3a4f0789
https://feedback.azure.com/d365community/idea/1f4f4333-c225-ec11-b6e6-000d3a4f0789
Hope this helps.
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