You need to be logged in to your tumblr account at this part.

When you visit the page you want the backlink from there's a heart symbol at the top of the page. That's what tumblr's like button is.

When you click that your "keyword-anchor-text" appears on the page to say "liked this"

cure-for-hiccups liked this

that sort of thing and the keyword phrase will be your link.

Because you have your account set to use a custom url that backlink now goes to your website rather than your post.

Refresh the page and you'll see your backlink which, because you have the redirect, goes to your site instead of your post.