I worked trough this topic, but none of the browser take the configuration.
The article is not clear it writes about User configuration, but documents Computer configuration.
I tried all 6 combinations:
HKCU + HKLM
SOFTWARE\Policies\Mozilla\Firefox\3rdparty\Extensions{446900e4-71c2-419f-a6a7-df9c091e268b}\environment
SOFTWARE\Policies\Google\Chrome\3rdparty\extensions\nngceckbapebfimnlniiiahkandclblb\policy\environment
SOFTWARE\Policies\Microsoft\Edge\3rdparty\extensions\jbkfoedolllekgbhcbcoahefnbanhhlh\policy\environment
In all tested browsers I need to configure the selfhosted server myself. I is not taken from the “base” REG_SZ in wether HKCU nor HKLM