From the XB1 dashboard, will have to sign in to see if you get it (probably for people who don't have gold atm), says any subscription so 1 month should be fine and considering Rocket League is normally $26.95 not a bad deal. Code will be sent to your Xbox Message Center by 31/7/2017.
If you rather not give your payment details to Microsoft there's this deal but it is 3 month gold sub only.
Interesting. Of the three accounts on my Xbox, two have no offer, but the third has a completely different offer:
"Minecraft + Gold
Buy 3 months of Gold and get Minecraft: Xbox One Edition, or buy 12 months of Gold and receive Minecraft: Xbox One Edition + Favourites Pack"