How can I set up a trunk that acts as a client to another PBX server?

I would like to set up a trunk that acts as a SIP Client to another PBX server which is NOT asterisk based. I can’t seem to find much information on what the “PEER details” options are in the SIP settings tab of the Trunks module. When I set up sip clients for that server I typically just give the domain (which is actually the IP of that PBX) and username and nothing else (it does not require passwords). How can I get my FreePBX to set itself up as a sip client in that way?