Unable to download stick of truth after preorder

Per the game's description, you should be able to download it for free when the new game launches.

Xbox support chat said I was to receive my code 7-21 days after preordering. Ubisoft site says the offer is for immediate download of Stick of Truth upon preorder

I thought that you could have access to the stick of truth if preordered before march 31st deadline

There is no deadline for it. Originally when the game was made available for preorder, the system worked differently and you could have downloaded the Stick of Truth at the same time as when you preordered the game. Shortly after that, the system was changed
to the current system (purchasing with currency is charged now, credit cards charged later) so the offer was updated to state that if you preorder the game, you will have access when the new game launches. This is from the game's description:

Pre-order now to get Towelie: Your Gaming Bud. For a limited time, get a digital copy of South Park™: The Stick of Truth™ on Xbox One free with purchase at the launch of South Park™: The Fractured But Whole™.

If the game does not show as free for you to download, then you will have to wait until the launch of the new game.

Technically you can't cancel a digital preorder but if you call support and they let you pretty sure the license for stick of truth will go with it