JIRA Update Snap - Mapper to add additional Labels to JIRA issues

Hello,

I'm running a JIRA Update Snap to add an additional label to JIRA issues based on a pre-defined JQL while preserving existing Labels. I have this working if I simply want to replace all Labels in my JIRA Update by mapping the String to $fields.Labels. But I dont want to wipe out the previous Labels, I want to preserve them and add the one specified in my Mapper. Here are some screenshots of the Pipeline and Mapper. 

What do I need to do differently?

Thanks,

Nico