I'm going to tell my ex's new boyfriend she was trying to get back with me
That's not nice. Why do you need to lie about that with your ex's boyfriend?
I want to piss her off
I understand that you are upset, but informing your ex's new boyfriend about her trying to get back with you is butting into their lives and impositional. Stay out of other people's lives when they are no longer in yours.
That sounds like one of those manosphere articles you read.
I do not know what a manosphere article is, but it feels like you're just going to meddle in their lives and I do not condone that. I hope you find someone else in your life that would make you happy, so that you can focus on her instead and not try to be vengeful with your ex.