You just need to learn how the syntax works for reference-style links. Something like:

[Hacker News][hn] is a website.

[hn]: http://news.ycombinator.com/

You should generally use these all the time when writing documents you intend to maintain. For quick one-offs like blog comments, you can instead use:

[Hacker News](http://news.ycombinator.com/) is a website.

Once you actually use markdown for non-trivial documents, it is very easy to remember its syntax.