Originally Posted by
Yoshi212
Currently the only foreign airlines is AC.
Even in the other scenario the person would get precheck when connecting to a domestic airline and only if they reprint their boarding pass (if they're selected to have it at all).
WS also participates in Pre-Check.
In addition to needing to reprint your UA boarding pass at a kiosk post-Customs, you will need to have entered your KTN in your UA PNR (preferably in advance) or be crediting to MP.