You need to sign up for a Total War Access account and according to the FAQ, you need to have the game installed on your computer, so I imagine launching the game and logging into the Total War Access account in-game to claim the DLC probably.
All you need to do is link your total war access account with your Epic account to claim it, then it gives you an Epic code to redeem. Little slow right now with server errors but it worked for me after a few attempts.
I managed to get the DLC. Of course, I still have to get much better hardware to be able to run the EGS because as is, it will not run on my integrated graphics.