You can use a special template if you need auto-assign high priority issue to some users.
When you select a template then it starts new automation with a filled-in answer for the automation name, trigger, condition, and action that accomplishes a specific use case. For high priority issues, you'll see "Advance compare condition" values in fields. The trigger is "Issue Created" and the action is "Assign Issue". You just need to select the assignee.
Enter the automation name and select the project. Don't forget to enable your automation.
When trigger and condition will fire then action will happen on the board.
Try the Automation Unlimited for Jira for free on the Atlassian Marketplace.