Post function to assign issue automatically to a user that is set within a User custom fieldAutomatically assigns the issue to the user in the selected user custom field.
To add Assign issue to the user in custom field post function, go to the transition of your workflow, switch to Post Functions tab and click on Add post function link.
Select JEP - Assign issue to a user in custom field and click Add button
Select the user custom field. The issue will be assigned automatically to the user in this field.
Click Add button and that's it
Do not forget to publish the workflow!