I'm currently working on 2 instances of JIRA software. One instance (cloud version) has the post function 'Copy Value From Other Field' which I use to pick up a user name from a custom field and copy it to the assignee field. Works like a charm.
The other instance (server hosted) does not have that post function. That instance was just updated to version 7.11.1.
Does anyone know if/where that post function is available on the server version?