goto account settings
userid from extension authentication settings in 3cx
password from extension authentication settings in 3cx
domain is ip or fqdn of 3cx server
authorization name can remain blank
Ive done this plenty of times you just seup an extension on 3CX for the phone and register the phone manually (using username and password, setting the registrar address as 3CX).
That being said why are you doing this and not using 3CX phone? It is far superior in every way.