you need the machine

Read an isekai's opening. Inexplicably, the hero found himself in another world. . . .

Nope. I mean, if the transition itself is not important, you have to downplay it. (Seriously downplay it.) But you need something. A machine, they used to call it. The character gets handed a lucky charm that lets you wish, and he wishes to leave. Or something.

Of course, if I wrote it, I would no doubt turn to the question of why such charms were handed out.