I don't think he gets to decide where he gets traded to, unless I'm missing an angle.
Because that along with blatant tampering is the only way I can see a logical tag scenario. Like if the Giants want him, but Darnold's agent has already made it clear he's going to the Raiders. Then the Giants might tell the Vikings to tag him so they can trade for him. Which obviously will end up being a bad look for both the Giants and the Vikings.
But still, I can't see any team wanting Darnold that badly. Even if the coach/GM like him, after his last two games there is going to be serious fan backlash for whomever gives him big money. Everybody was watching those games. If he was paid the same as Rodgers/Cousins/Wilson, the fans would be on board, but if it's 2x their salary at 3x the length, it will not be a popular move.