The button for taking photos and it linking up with the missions is the same button you press to take photos in any game on PS4, it’s the share button.
But its the default press, which I believe is either a double tap or a long hold.
I have my screenshot capture set to a single tap for photos and that doesnt work for me on the photo missions because it’s not the ps4 default, I have to hold the share button (or maybe it’s double tap, it’s been a while. No photo mode in VR.)
I hope I’ve understood correctly and you don’t mean X as in the button on xbox controller. But I would have assumed it wa the A button to take a photo on that console.
This is how it worked before beyond, it’s possible that’s changed since next but I’ve yet to play a non VR session.
Hope this helps you.