How do I automatically assign Tester.

[Sam Yuan Ng]

I am on a team manage project. 

I am looking to set an automation whereby when any one transition an issue from development --> Testing. The automation will automatically set my tester to a particular person (A particular QA). 

I am trying the edit issue function on automation using advance field settings. But to no avail. Not really sure which Json code i should use for this. Please advice. 

Not able to use assign issue as that only allows me to change the assignee from the developer to someone else. (Not my intended action to change the assignee but the tester which is in a separate field instead)

Thank you