Open the SAP Connection object used by your SAP Agent.
Go to Remote Function Call tab.
Enter the missing parameter value (in this case, Application Server).
To get an SAP-Server working with a Message-Server, an additional entry has to be made in the file /etc/services (Linux) resp. C:\Windows\System32\drivers\etc\services (Windows), please see SAP Message Server Parameters, list item rdisp/msserv for more details.