Example: Add watchers from a part of the issue summary: "Summary_text - watcher1, watcher2, watcher3, ..."
As in my answer to your previous question you should move the post-function below "Creates issue originally".
Use post-function Regular expression renderer to remove the part of the summary preceding the comma separated list of users. We use "Ephemeral string 2" as replacement string because it isn't initialized. Regular expression utilized is "^.*-\s"
Use post-function Add or remove watchers to add users in "Ephemeral string 1" as issue watchers:
Once configured "Create Issue" transition looks like this: