If you wish to use your current Bungie.net profile, all accounts linked to it will need to be authenticated in order to be able to cross-save. This means that in case of forgotten login credentials, you'd need to contact the platform in question for potential means of recovering it. In your case Xbox support. If you are unable to recover the account, you'll need to create a new Bungie.net profile and bring your active accounts to it. Note that doing this will cause you to lose any items redeemed using your current profile, for example, promotional emblems.