I tried to do this, so I could write you good instructions, and found that "Labels" isn't one of those fields you can auto set with a post transition on the Create screen! (That's how you'd do it for other standard fields and even custom fields via a plugin.)
Next, I did a quick Google search and found another Community post which seems to agree: you can't do it natively, but the plugin called "Script Runner for JIRA" might offer the ability.
Otherwise, this sounds like a feature request for Atlassian! You can log those at: https://jira.atlassian.com.
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