Here are the steps on adding a ProxyServer url and Port using Group Policy Preference Registry settings.
Requirements: GPMC.MSC Editor
NOTE: Adjust the Action option bases on your needs.
In our scenario, we are setting the following:HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Internet Settings ProxyServer entry
Here is what it looks like, once it is configured:
That is all, once you have defined this GPO and your Domain replication is completed or use GPUPDATE /FORCE from the client to see if this setting have come down, you are done!
For references to GPO Location, use the online GPO Search Engine: http://gpsearch.azurewebsites.net