...
Click the Jira cog icon > Issues > Workflows.
After finding the workflow, go to the three-dots icon and click Edit. Navigate to the transition of your workflow.
Switch to the Post Functions tab.
Click the Add post function link.
Select JEP - Add Subtask User(s) to Parent Issue Watchers List Post Function and then click the Add button.
Fill in the form:
Choose a field (Assignee, Reporter).
Choose a Custom field (all single or multi-user custom fields are listed here).
Click Add, and you’ll be redirected to the Post Functions list.
Don’t forget to click Publish Draft to publish the workflow and save your changes.
...