First option - guaranteed to work:
Book any roundtrip flight with it for latest possible date next year main cabin (not saver) - e.g. SEA-PDX.
Once you know where you will be flying, make a free change (fare-difference applies). This means you must travel sometime in 2022 to use the certificate.
Second option - not guaranteed to work:
Book any roundtrip flight with latest possible date (not saver) on a route that you think will eventually be on the chopping block and will be eventually cancelled e.g. SFO-DCA or something.
Hope it cancels. If it does cancel call alaska airlines and beg them to reissue the certificate, even if its not officially allowed by policy.