I pretty much described exactly what I need to do, what information do you need?
I can run the following command in a local, administrative CMD and the registry key /value is written correctly:
reg.exe add "HKEY_LOCAL_MACHINE\Software\Microsoft\Office\ClickToRun\Configuration" /v UpdateUrl /t REG_SZ /d "my-test-value" /f
I can create a batch-script with the mentioned command and execute it locally. the registry key updates correctly.
When I use this batch in a pre-installation-activity in software deployment, the registry value does not change.
The script works locally but not when run from pre-install-activity.