How do I donate to the creator of Hannahs_Messy_Handwriting?
Anyone know how to contact her? Or where on the site I can donate?

You will find the author's email address at http://www.dafont.com/profile.php?user=249893 You can also send her a private message by clicking on the "Send a private message" link also on that page.

Get in touch with her and ask her where to send the donation. Also, it would be better for you to ask if she has an amount in mind for the donation.