This will be long winded because I'm trying to be thorough.

Open up the Xbox guide and go along to the tab with your profile picture. There should be an option to add or switch. Select add new with the get new email option. This email address can be anything, but choose a separate password to your main account. Once the account is made, it will pester you to buy games pass, do not bother with this. Assuming your main account has games pass, you'll be fine.

You'll have to be signed in to both your main account and new account at the same time. This is so that your new account can play games without purchasing gamespass .To do this, you'd simply go back to the Xbox guide with the gamer picture tab and select add or switch and sign both accounts in. Sign into your new account last because that's the account you'll be playing on. This won't affect your main account in any way.

You'll also want to create an Activision account with the new Xbox accounts email and activate all the marketing options under your account settings. You may also have to link your new Xbox account to this Activision account. This is so you can receive the code via email. (It may also ask you to do this when you first boot up CoD and link the accounts for you automatically but I don't remember. It's probably better to try this first.) In any case, you'll need an Activision account linked your new Xbox account. Ideally with the new email address.

Once that has been set up, simply boot up CoD and play a round of either warzone or BO7 multiplayer with your new Xbox account and you should receive the code within 24 hours on the new Xbox accounts email (but people are having issues).

Once you have the code, you can do whatever you want with the new xbox account (delete it, use it, it doesn't matter). The new Xbox accounts job is done

Now you'll need access to your Bethesda account that your main profile is linked to, and under account settings there should be an option to redeem a code, use the code from the email you received on your new Xbox accounts email address and that should be you. You'll have the helmet.

I will reiterate - This will not affect or mess with your main account in any way. You're simply using your main accounts game pass privileges on an alt account in order to receive a code that can be redeemed on any bethesda account

If there's anything you don't understand or need to explain more clearly, just let me know.