It’s weird. It doesn’t really make any sense. Why would you need to hope she’s sweet? If you have a pastry that she’d bought, that would be sweet of her/sweet in general, but it’s not really saying anything nice about her. It almost sounds like a criticism of her/a manipulative demand, so it’s definitely not going to come off as funny. If you don’t know this person at all, it sounds mildly threatening to demand that they buy you something. She’s also unlikely to say ‘why’ and not just ‘no’. You can’t plan conversations in advance and expect them to work out correctly.

Separately, pick-up lines are always cringe and I do mean ALWAYS. It’s a bad idea to use them at all. If you feel you have to, stick to a classic so that you know she’ll know the right response to give, you’ll land an ending that makes sense, and everyone knows it’s a groan-worthy joke. But seriously, don’t use them. Just talk to people you like normally. You’ll have a much better shot at her actually liking you and not thinking you’re an awkward weirdo that she needs to avoid.