From OrientDB manual:
On first call (or when the session is expired and a new authentication is required), OrientDB returns the OSESSIONID parameter in response's HTTP header. On further calls the client should pass this OSESSIONID header in the requests and OrientDB will skip the authentication because a session is alive.
You can retrieve the OSESSIONID parameter by retrieving the HTTPHeader list from the HTTPResponseObject and then apply list operation find where you search for the key (which might be OSESSIONID, but that you would have to doublecheck).
You can set a new custom header when sending a follow up request in the Call REST service microflow activity.