Hi there,
I have checked the community already and found some entries on the same issue.
It seems an easy setup:
The "Edit issue" function let's you add a json under "additional fields" without selecting something using the drop down. Here we use:

However the request participant filled won´t be field with anything - without any errors so I am not sure where to start troubleshooting (FYI I have no experience with JSON so just adapted the information from other issues and made sure to close all brakets )
btw at absolut best the user should be something within smart value! However need to know format used in case you suggest something 
cheers
Felix