As per the title, how do I get Staad to analyse a connection as a pinned joint?
You may assign "member truss" to any member, then pin will be provided at start and end nodes of the member.
If you want to make a joint as pinned rather than setting truss property to the members, you may release the moments (MX, MY and MZ) at the node on all members which connect to it. Choice of the node (START and END) for the moment release must be done carefully verifying the orientation of the member
Arunkumar