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.
Hi Everyone! My name is Mina and I am on Atlassian’s Ecosystems Marketing team. Our team is focused on our technology partnerships and marketplace apps. One of Atlassian’s partners is Slack, who ...
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