Hello Lassi,

You should create a new Business Service class extending the HL7 Business Service of your election (TCP, HTTP, etc) and after overwrite this method:

method OnConstructReply(Output pReplyDoc As EnsLib.EDI.Document, pOriginalDoc As EnsLib.EDI.Document, ByRef pReplyCode As %String, ByRef pSC As %Status, pEarlyAck As %Boolean) as %Status

Override this method to construct a customized reply document. Inputs are the original document and the reply code and the status code that the framework produced in processing the original document. If you change the status code but do not construct a reply document, the framework will use the new value to construct a standard reply document. 
If you construct a non-HL7 object it must still have a property called 'Envelope'. Piece:2 of this value will be written verbatim.

Consider this example (I didn't test it):

method OnConstructReply(Output pReplyDoc As EnsLib.EDI.Document, pOriginalDoc As EnsLib.EDI.Document, ByRef pReplyCode As %String, ByRef pSC As %Status, pEarlyAck As %Boolean) as %Status

{

    Set pReplyDoc=pOriginalDoc.NewReplyDocument(,..LocalFacilityApplication)
    Set pReplyDoc.Source=pOriginalDoc.%Id()
    Do:..#UseOriginalControlId pReplyDoc.SetValueAt(pOriginalDoc.GetValueAt("1:10"),"1:10") ; copy the control id to the ack control id
    Set tAckMSA=##class(EnsLib.HL7.Segment).%New($LB("",1))
    Set tAckMSA.Separators=pReplyDoc.Separators
    Do tAckMSA.SetValueAt("MSA",0)
    Do tAckMSA.SetValueAt("CE",1)
    Do tAckMSA.SetValueAt(pOriginalDoc.GetValueAt("1:10"),2)
    Do:$G($$$ExpectedSequenceNumber) tAckMSA.SetValueAt($$$ExpectedSequenceNumber,4)
    Do pReplyDoc.AppendSegment(tAckMSA)
    Set pReplyDoc.IsMutable=0
    Quit $$$OK

}

Hello Fabian,

You can get it from WRC, go to the Online Distributions options, choose the Components option and after filter using 'node':

Also, maybe my post Create a Node/Express BackEnd and connect it to IRIS in less you say 'Mississippi' can help you a bit.

Regards