If you purchase an Appsumo code you need to create an account here : https://useartemis.co/appsumo to enable your first code.

If you already have an account and want to use the same email address, you first need to delete your existing account.

To do that you just need to go to settings > Account > Delete account.

If you already have an Appsumo code registered to your account and want to stack another one, you can do it by going to > Billing and add your new code there.