One person in you instance is the authorized support admin and can add other instance users as support contacts. If you do not know who that person is or if that person has left the company, email to email@example.com
There are a lot of issues with the mechanism, though, from my experience. I often cannot access support on my customer instances and it takes forever to get it fixed. (OI have case as with speak that has been lasting since April...)
Hi Robert Pugh, in the support portal where you submit cases you should see a tab to manage authorized contacts and then there will be a list of your users who you can select from. If you don't see this tab, you might not be authorized to select authorized contacts, but you can fix that by opening a support ticket.