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.
Hello, Community! My name is Gosia and I'm a Product Manager on Jira Server and Data Center here at Atlassian. Since 2002 when we launched our public issue tracker, jira.atlass...
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