YY, write-ins for presidential races aren’t so simple as they are for other races. You are not voting for president, despite the popular misconception that people vote for president. You are voting for members of the electoral college. And the members elected to the electoral college will vote in December who should become president.
So a write-in vote would only be for membership in the states delegation for the electoral college. If you write in McMullin in most states you aren’t really voting for him since he doesn’t have a recognized slate of candidates for the electoral college in most states.