Oh, I just recalled a reason for why there isn’t a socket already and cannot add one.
This is a legendary upgraded from rank 1 purchased prior to 9.1.5 when it would have included a socket.
Doubtful, as all crafted gear is a season one base that can be upgraded, when crafting, to the current max level for such.
I believe that upgrading, since you need the base item to upgrade, would retroactively add the socket to the piece. what the OP likely has is: https://www.wowhead.com/item=172256/shadebound-waistguard?bonus=6893 upgraded with https://www.wowhead.com/spell=359669/crafters-mark-of-the-first-ones
can you use the creation catalyst on the belt? if so, then maybe that conversion can then accept the socket. 
I tried with ENG goggs, no go.
Where do you buy that?
From the vendors near the guild bank in Oribos. Here’s what was update in 9.2:
they probably consider crafted gear to be season 1 even tho you slot a season 3 upgrade into it.
Ah, ok. Thanks. I don’t do the vault stuff so that’s why I didn’t know about that one.