It's on the reply thing, like the earth with the sideways 8, next to the emoticon list. Press it, and it should be self explanatory. Just highlight the word that you want to turn into a link, and it'll be turned into a link.
1. Highlight the text or image you want to be linked.
2. Click the globe with the chain link on it (between the smilies and the envelope button).
3. Paste your link the the pop-up that will appear after pressing the button.
4. Press OK and BAM, you just got linked.
Or, yeah, just type in the code. I do that sometimes on mobile if the buttons are being weird and my fat fingers are being extra fat.