Original link: https://www.youtube.com/watch?v=t2aFykDs__c
The process is exactly the same for any game. Only the host needs to own the game and configure controls, client only needs to press two buttons in Parsec and is already playing with host. Parsec basically gives a virtual controller to the client and displays the host screen in his machine.
Unfortunately, I can't use it because my GPU is super old, but I'm spreading the word because I saw this being used so well in other Mario fangames and games like Cuphead, SMBX deserves to be played online. Battle levels and coop oriented episodes can finally rise.