In this example the assignee shall be set when transitioning a ticket from "To Do" to "In Progress" based on a user picker custom field. When there is a user in this field he should be set as assignee, otherwise the reporter should be set.
For this a user picker custom field and the post funciton Set a field as a function of other fields are needed.
The to be parsed is:
(^$)%{00006}
(.*)%{10405}
Note that:
Once configured, the transition will look like this: