What others seem to be teaching is just to go in and add your url in order to get a backlink.
I'm a firm believer in earning them though. After all, tumblr.com is a fair sized site and these methods won't be around if folks keep spamming them.
So for my backlinking, like wealthy affiliate teaches, always provide value.
So instead of putting in just your keyword. Actually create a post.
Others are trying this and not putting any content onto the site of value. Even it's an opinionated post, it's something that if someone turns up at the post, they can find something to talk about. Leave a comment etc. That's the sort of stuff that keeps sites online.
So go into your page and create a post.
Just like any other directory or blog.
Click the text icon at the top of the page.
Create your post.
Get your keyword in your title, first paragraph and last sentence.