Offline Extension install in Edge

I try to install an Edge extension offline (from a File Path) with these GPO Settings:

Windows Registry Editor Version 5.00  
  
[HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Edge]  
  
[HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Edge\ExtensionInstallForcelist]  
"1"="gbchcmhmhahfdphkhkmpfmihenigjmpp;file://C:\\temp\\Extensions.xml"  
  
[HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Edge\ExtensionInstallSources]  
"1"="file://C:\\temp\\"  

The XML Contains:

<?xml version='1.0' encoding='UTF-8'?>   
<gupdate xmlns='http://www.google.com/update2/response' protocol='2.0'>   
  <app appid='gkemdbggknnnkjppcjmblecldminppbf'>   
  <updatecheck codebase='file://C:\temp\extension_10_0_12_0.crx' version='10.0.12-stable' />   
  </app>   
</gupdate>  

Still no success after many different tries (ExtensionInstallSources i added later)... anyone succeded with this?

Also tried other File URI Scheme:

    Windows Registry Editor Version 5.00  
      
    [HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Edge\ExtensionInstallForcelist]  
    "1"="gbchcmhmhahfdphkhkmpfmihenigjmpp;file:///c:/temp/Extensions.xml"  
      
    [HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Edge\ExtensionInstallSources]  
    "1"="file:///C:/temp/*"  

Also tried this:
https://learn.microsoft.com/en-us/microsoft-edge/extensions-chromium/developer-guide/alternate-distribution-options

Unfortunatley there is never any mentioning of any local path...

Thanks!