Danbooru uses an awkward way of linking. Put the words you want to appear in full quotes, then a colon, then the URL with no spaces.
"Such as this one":http://danbooru.donmai.us/posts/726274
Alternatively, posts on Danbooru and its mirrors will autolink when you plainly type, for example, post #726274. The benefit being clickers will stay on their domain instead of being transferred to the one you use.
Edited because comments now have auto-formatting that ruined my explanation...on top of now accepting multiple kinds of link formatting to begin with.