This implies adding a new link input for agent and there are several file locations. You must use existing code as example to create new one. This will imply PHP, HTML and CSS knowledge.
Hope what we found below helps:
I did a theme search by agent_facebook which is the name of the field used for facebook login in agent unit. And there are 23 results you would need to look at – and replicate the same for AGENT and a new link.