You can use FIELD :
ORDER BY FIELD(ticket_priority_id, 'ASAP', 'HIGH', 'MED', 'LOW')
Please note that if ticket_priority_id not in the following list, it will return 0, so you might want to add this as well:
ORDER BY ticket_priority_id NOT IN ('ASAP', 'HIGH', 'MED', 'LOW'), FIELD(ticket_priority_id, 'ASAP', 'HIGH', 'MED', 'LOW')
put lines that have no priority at the bottom, and not at the top.
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