JIRA Cloud - in automation, how to add a new label, not edit the label

Adding one nuance to Chris's answer above, since it's specific to the visual editor and there's a common follow-up gotcha:

If you ever need to switch that same Edit Work Item action to the Advanced / smart-values (JSON) editor — for example because the label needs to be computed dynamically rather than picked from the field list — the behavior changes. In Advanced mode, "labels": ["new-label"] genuinely does overwrite the array; there's no implicit "Set = merge" behavior there like there is in the visual field editor. To append instead, you need to reconstruct the full list explicitly:

{{issue.labels.add("new-label")}}

or, if you're building the array from scratch:

{{issue.labels}}, new-label

(then split/rejoin as needed depending on how you're assembling the JSON body).

One more thing worth knowing if multiple automation rules touch labels on the same issue: even the visual editor's "Set" is a read-then-write of the whole list under the hood, so two rules firing close together on the same issue can still race and clobber each other's label. If that's a realistic scenario for your rules, the explicit ADD/REMOVE mode Chris mentioned is safer than Set for exactly that reason — it's additive rather than replace-the-whole-list.