Register new anonymous user with specific role through rest api in Drupal 8

I'm using Drupal 8 rest service to register a new anonymous user and it works fine; the user is created in Drupal.

Then I tried to specify a custom role sending the request below

{
    "name": { "value": "FooBar" },
    "mail": { "value": "[email protected]" },
    "pass": { "value": "secretSauce" },
    "roles": [
        "my_role"
    ]
}

but I receive the error

{
    "message": "Access denied on creating field 'roles'."
}

I checked permissions but I'm not getting what is missing.

Solution

An anonymous user cannot assign themselves a role. Drupal automatically assigns a user the role of 'Authenticated user'. You can alter the permissions of that role accordingly if that suits your needs.

*If you are using the excellent Simple oAuth module to authenticate, then you can also assign the user being created via the REST API a particular 'Scope' (role) based on the consumer ID used when creating the account. On your app you can then create the logic for the particular role to be assigned to a particular user.

*This all assumes that you are using a headless approach and the users are being created from a JS front-end.

EDIT

Create custom field for Drupal Users
Install the Rules Module (https://www.drupal.org/project/rules)
Create a rule to act on a new user and assign the appropriate role per the value of the custom field.
Pass in the desired value for the custom field from your front end (like your example above)