The answer was neither in the book nor in the show yet, from screenrant
In Sapkowski's written version of "The Last Wish" Yennefer addresses the wish immediately after they are spared death from a crumbling ceiling (a home in the Netflix series, a tavern in the book). Saying "I heard what you wished for" and that Geralt's decision "astounded" her, it's a major part of what sparks their session of lovemaking. When they finish, Yennefer revisits the scale of his wish... and what it may cost him:
'Your wish,' she whispered, her lips very near his ear. 'I don't know whether such a wish can ever be fulfilled. I don't know whether there's such a Force in nature that could fulfill such a wish. But if there is, then you've condemned yourself. Condemned yourself to me.'
The Netflix version can't communicate Geralt's words loud enough for Yennefer to hear without the viewers also knowing, which is important. The books themselves never explicitly confirm the truth shared by Geralt and Yennefer, but they do explain the problem more than the show does. With Geralt's life in danger along with Yennefer's, a wish can only be used to save one of them. Unless Geralt gets clever.
But for the show:
Which is when the priest drops the closest thing to an explanation fans may get, saying:
"It's not that simple... But if... If he expressed the right wish... If he somehow tied his fate to the fate... No, I don't think it would occur to him. And it's probably better that it doesn't."
Most fo speculation is he tied their fate or asked for the kid with her etc as said in another answer.
For the word of god from TV Guide
"We know," she confirmed. "I'm not sure anyone else will ever know.
But to know the story, we had to very much decide on, 'This is what
we're hearing,' so that we can make sure that as the dominoes fall in
later seasons we all understand what it is."
When asked whether the audience would find out what Geralt wished for,
Hissrich played coy, saying that she likes "to let the audience keep
guessing on that one" for the moment. However, Anya Chalotra is
confident that the series will reveal what Geralt wished for...
eventually. "Absolutely. Yeah. I'm sure," she said.
So we should be patient and see.