Click on "assignee", or use a post-function that sets it to the reporter on a transition.
JIRA doesn't naturally set the assignee when changing issues though, so when you say "instead of default assignee", you are referring to something your admins have set up for you, and that's what needs to be changed.
I’m a designer on the Jira team. For a long time, I’ve fielded questions from other designers about how they should be using Jira Software with their design team. I’ve also heard feedback from other ...
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