In this example, the assignee shall be set according to a user picker custom field "Testers" when transitioning a ticket from "To Do" to "In Progress". If a user is set within the field, he should be set as assignee, otherwise the reporter should be set.
This can be done using the Set a field as a function of other fields post function.
The to be parsed is:
(^$)%{00006}
(.*)%{10405}
Note that:
Once configured, the transition will look like this: