I have checked in the store and searched from the xbox app and cannot find the expansion.
I don’t think there is a way, sorry.
It may be pre-loaded already, it at least some of it. The content patch on last Tuesday for the car pack, probably contained a good bit of it, since it was significantly larger than the last car pack related patch.
It will not appear until tomorrow, even though yesterday i could redeem a free Blizzard Mountain DLC promo code from the Racing Championship stream I dont see it the store either 