How to add an additional search filter for tickets in the Self Service Portal?

I am looking to add an additional search filter to the self service portal besides 'status', 'urgency'
and 'priority', because our users have had difficulties finding back tickets since the transition to the self service portal. We were informed that adding in a custom filter was possible during one of the Sysaid webinars.

Any direction to documentation on how to do this or additional guidance on how to find back tickets for our self service users would be very helpful. Images for reference: