This post function automatically assigns the issue to the user in the selected user custom field.
To add the Assign Issue to User in Custom Field Post Function
Go to the transition of your workflow, switch to the Post Functions tab and then click on the Add post function link.
Select JEP - Assign Issue to User in Custom Field Post Function and then click the Add button.
Select the user custom field. The issue will be assigned automatically to the user within this field.
Click the Add button and that's it.
Do not forget to publish the workflow!