I think in the past it was part of the API data record, so take a look there. But no matter what, chances are high you have to get your boarding pass at a check-in desk in SFO anyway. Then the agent can add the TSA-precheck there without problems. Mobile boarding passes ex-US never worked for me on SK... Or to be more precise.... not in the last few years... used to work pretty ok in 2015-2018.