Add @username to Tweet button – I'LL PAY FOR HELP, I'M THAT DESPERATE

How can I add “via @twittername” to the end of the message when someone shares my post? I’m using WordPress’s default sharing (JetPack), no plugins. I’d love to edit the message so I can shorten the link too instead of the long URL on Twitter, but keep the other networks the same.

So, Twitter goes from “Post name: LONG URL” to “Post name | Shortened link via @twittername”. All my other network sharing is fine.

THANK YOU!