There is an action to add (and remove) a label from an email; however, there isn’t a good explanation of what is expected in the “Message to be labeled” field. What is the syntax this field is looking for?
Best answer by Troy Tessalone
The ‘Message to be Labeled’ field expected the Gmail Message ID.
TIP: You can determined the expected value for a dropdown field by viewing the available options, which tends to be a unique system ID (but not always) shown as the smaller gray value underneath the friendly value.