How does a presidential candidate get drafted?

Answer 1

First, there would need to be a national draft.

Second, they would need to be young enough to fall within the statutory age range for the draft. Given minimum age requirements for president (in the US) that is very unlikely.

Answer 2

By the media. They claim people are demanding someone who does not plan to run is running. The media then claims they are being drafted. If you only knew - the "draftee" already has a committee, a staff, and is out there running for office behind the scenes, but it sounds good if they "drafted" to run.