Play into her curiosity

You looked down at the pokeball, then back to her. "Are you sure you want me to?"

May frowned at you, but she looked more confused than angry. "What do you mean am I sure? Of course I am?"

"I mean, how many other humans have ever had a chance to experience this? You admitted yourself that I could have talked you into giving it a try-"

"If you convinced me it wasn't an obvious trick." May interrupted.

You nodded. "If I convinced you of that. And right now, well... To put it bluntly, I wouldn't need to trick you right now. I caught you and we're all alone."

May raised an arm to her chest defensively. "What are you saying?"

"I think we're both curious to see what it would be like for you to... to sort of be my pokemon. And you can trust that it isn't a trick, because I don't have a reason to trick you under the circumstances."

"That would be stupid..." May looked away, not sounding convinced of her own words. "I'm a person, not a pokemon."

"Pokemon can be people." You pointed out. "And obviously I didn't mean it literally, sending you into battle against a pokemon would be stupid." May bit her lip and didn't answer. "What would it hurt to try? You can always back out later."

"... Al- No, wait." May looked back at you. "What would this actually mean? What are you asking me to do?"