Post a Message to chat service - how to include custom fields in the message

[Margaret Boisvert]

I am using the Adaptavist Script Post-Function [ScriptRunner] post function - Post a Message to Chat Service using Slack.  I am able to create and send a message (YAY!) but I would like to include the values of some custom fields and I just can figure out the proper notation to include them.  I am using issue.summary and issue.description without issue but I want to include a field called Severity and I can't seem to get to that.  I have tried using ${issue.Severity}, ${issue.severity}, ${issue."Severity"}, ${issue.fields.severity} and possibly a few more but none with success.

Can anyone help me get over this hurdle?