To set this option, please tap More... | Accounts | choose account | Advanced Account Settings | Always BCC | Enter your own address.
You can do it for one or more of your accounts to give yourself maximum flexibility.
On iOS
To set this feature up, please go to Settings | Accounts | choose your account | Always BCC | Enter your own address.
On Desktop
To set this feature up, please go to Settings | Account Settings | choose your account | Always BCC | Enter your own address.