How to confirm the registration and set initial Password

Hello there,

I am implementing open registration. Now the user gets the code.
Want to use my own ConfirmationForm.
I posted to..... /api/v2/user/register?login=false
{
"email": "name@provider.com",
"new_password": "Password",
"code": "codefromMail"
}

Response

{
  "error": {
    "context": {
      "email": [
        "The email has already been taken."
      ]
    },
    "message": "Validation failed",
    "code": 400
  }
}

What am I missing?