If you want to write a script in Jira, then you would need an add-on like Power Scripts or ScriptRunner.
If you do not want to buy add-ons, you can develop your own add-on.
If you want to auto set some fields when a ticket is created, it is not necessary to write a script, you can use the Update Custom Field Post function, or use Automation for Jira add-on.
Thank you Alexey
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