[Freeswitch-users] How to add headers to an outgoing REGISTER

Hello, I have read the documentation under https://wiki.freeswitch.org/wiki/Sofia-SIP#Adding_Request_Headers and as I understood it is possible to add Headers to SIP requests simply by changing the configuration. But the documentation refers to the dialing plan. I tried to add headers to an outgoing REGISTER but failed. Because the documentation refers to the dialplan and changing headers for existing calls, I am not sure if adding headers to a REGISTER request is possible or not. Can anyone please tell me if it is possible or not and if yes, how to formulate the correct "<action application="set" .>" and "<condition>" sections in the configuration. Thanks in advance for all answers. Best Regards, Lars