Custom script to append an ID number to syslog event?

I am new to Kiwi syslog and don't know much about using Jscript.  I'm reading that I need to create a script if I want a custom field added to my custom file format.  I wanted to do a simple task of appending a specific ID number at the end of each event that is written to the syslog file.  There is a repository that I send my syslog files to but the parser for that system needs the specific ID for my system to be at the end of each event message within the file.  This is not the correct syntax but I want to do something like the following for example:

original message would look like = 2018-Jan-4 19:37:17 host IP 10.1.1.1 event message

modified message would look like = 2018-Jan-4 19:37:17 host IP 10.1.1.1 event message SystemID:12345678987654321

Function Main()
    'Text to append to raw message
    appendID = "SystemID:12345678987654321"

    'get the raw message
    modifiedRawMessage = Fields.VarRawMessageText
  
    'Append text to message
    modifiedRawMessage = Append(modifiedRawMessage, appendID)

    'Overload message text with modified one.
    Fields.VarRawMessageText = modifiedRawMessage

    'Return success
    Main = "OK"
End Function

Can someone help me with getting the syntax correct?

Thank you in advance.