I am trying to give the customer ability to raise the priority of the ticket from their view. How to accomplish this capability?
How to customize the Customer Support Portal
It's not best practice to let the customer select the priority of the ticket as they will always use the highest priority rather you can based the priority on the ticket type they will be selecting from the end user form.
In this way you know what's the concern/ticket type and set priority on each ticket type.
I agree with Jahn, but I think you can simply change the visibility of this field from Admin Center if you really want to make that change:
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