How do you chatlink something you don't have?

There are 2 ways that I know of:

The first is to use the mod AtlasLoot, the second is to click the "Link" button on a WoW Head page.

Personally, I use the addon Ludwig, which allows you to do a simple search for any item that your character has seen by name.

Well, if you have that profession, you can link it directly from there. But there are several addons that will collect links that you've seen in game. I use Linkerator. With that, you can type /link (see the help/readme for options).

For example, if you want to see the mats for making Brilliant Spellthread, you could type

/link spellthread

and you would get all of the types of spellthreads, they're item links, spells, and the tailoring recipe (which would show you the mats).

I don't know of a way to do it without an addon though.

I wanted to add that if your main interest is in crafted items, TradeSkillInfo provides a lot of information on each of the professions - and you can link the recipes or the items from in there directly.

You can also simply look the item up on AH, and link from there. Or if it's a tradeskill related item, you can link from your profession window.