Add/remove Watcher (via Custom Field)
With this post function you can add or remove watchers from a (multi) user picker custom field. This has been a highly requested feature since 2004, see JRA-5493.
Usually setting watchers is a manual step after the issue creation or workflow transition. With this post function you can set them in one single step.
You can combine this feature with our required fields validator and force a user to name a watcher. For example you could set a field "Reviewer" required so that a user must nominate a reviewer in a certain workflow step.
A common use case is to combine the functionality "Add Watcher" with the "Default Values for Create Issue screen". That way you can predefine default users which should be added as watchers to the issue in a defined custom field (multi user picker). On the "Create" transition you can add the post function "Add / Remove Watchers" and all the selected users in the custom field will be automatically added as Watchers.
Note: If the user already exists in the watcher list, he won't be added twice. In addition the post function will add only those users that have the permission in that project.