Creates or associates an account from your system to a Privakey account.
If the accountId has not yet been used, a new account is created and its Privakey GUID ("privakeyId") is returned. The location URL of the new account is placed in the header. Additionally, a "sessionToken" is returned. Both the privakeyId and sessionToken must then be passed to the device the user is trying to register.
If the accountId has already been used, its Privakey GUID is simply returned.