How can I link to an item I don't have in the chat in World of Warcraft?

On Wowhead, there is a "link" button on each item, glyph, weapon, etc. page. This link will provide you with a "Script" that will look up the item for you in the game.

You can find it in the top right corner.

If you copy the text, and then enter it into your chat box in the game, it will put the item link right in your chat window. Then, you can shift-click to link it to the crafter.

You don't need an addon of any sort! Since you've specified WoWhead, you've made it quite easy on yourself, as WoWhead is conducive to these sorts of things!

First, navigate to the item in question: I used "Perdition's Blade". Second, look for the small "Link" button, circled, below, in orange:

The following text-box will pop up.

Copy the string, and paste when you're in-game. Voila.