Now taht we are at it, Márky-Zay is the PM candidate of the opposition. Which means not much, since we don't vote for PM candidates, but for:
1. a party list
2. a representative (for the legislation; he doesn't even have to be in the same party which we support)
Regarding to parties, on the ballot there will be the Fidesz-KDNP, the Mi Hazánk (right radical, from the Jobbik), and an opposition party depending on which one is the "strongest" locally - so probably not a common party alliance of the coalition. And maybe a couple of small parties, like the Two Tailed Dog Party.
Then after elections, the new legislation elects the PM. Which literally can be anyone they want if the opposition wins, it's just an agreement (supported by the voters in the primary) that Márky-Zay will be the PM. If the situation will demands or allows it, it can happen, that someone else will be that. Ofc if they disregard the primary arbitrarily they could lose popular support, but I'm not sure how much that matters. And since the whole parliament elects the PM, the opposition parties probably won't have the luxury to vote on the person for PM differently - Fidesz might be in there in large enough numbers -, so they'll stick with Márky-Zay.
And ofc this is all just hypothetical, last I heard the Fidesz leads now.