When you pre register your clan, if your name is not taken already, i see no options to set the Four letter clan tag. They say COD elite is totally apart from ghosts, so what does that mean for clan tags, do they simply carry over what your elite tag was? They say to get a gold clan tag you will have to rank up your clan, i wonder if that is how choosing your clans tag will work as well. you just get to choose when your clan ranks up?
Plus there is an Elite Forum.. Plus there is a huge sticky about Clan recruitment/issues at the top of the Forum page.
The short answer to your question if it works like Elite did, you won't have the option to set your clan tag unlocked until you reach level 10. That's how it was in MW3 but in BO2 things didn't reset, so you got the advantage of being able to keep what you had unlocked in MW3 and carry it into BO2. The good news from what I've read so far is that the clan tags won't be unique anymore. So you won't have the issue of not being able to get the clan tag that you want do to some other clan locking it in before you. Of course, this may change but I sure hope that it doesn't.
Go to My Clan on the COD app. Then press the Helmut with the gear in the upper right hand corner (this is the edit button). This will take you to a page that shows a section for the clan tag. Press that, put in your tag and press the save icon at the top of the page and you're done.