probably obsolete by now with the patchesSolution for those who have game share
I'm posting this because, even with the updates from Reaper R66, there's some confusion within the comments.
Use the general guidelines (match set up and the items) provided by his solution. Check the items used on a side note.
I had to try a few times, but this is the way to get it done if you share games with someone and if you are not using your Home Xbox
(even if it is):
1 - you may be online, doesn't matter;
2 - make sure that Rocket League is closed
(and not at that minimized state);
3 - hard reset your Xbox One
by holding the power button for 10 seconds;
4 - when it boots, sign in into the other account
(the one that you share your games with) and boot into Rocket League
5 - press any button and, by pressing the back button once in the main menu, switch profiles to the one with your gamertag
6 - from there, choose: exhibition, rumble, unlimited goals, any arena, and a rookie bot;
7 - start the match and check the power-ups that you pick.
Hope this helps, it was the way I did it since I use game share and my Xbox doesn't count as my Home Xbox - besides, and to make it worse, I'm the one who bought Rocket League, so there's no way to try the offline method.
Thanks for Reaper R66
and his well-crafted solution, I just wanted to make this right away visible to those who fit into description.
Extra thanks to left 4 zaneage
for suggesting a more detailed solution.