@alex-patterson The error was just simply 401 Authorized ....

I was also following this guide from the API docs:

Retrieve a User Registration
This API is used to retrieve a single User Registration. This is the information about a User for a single Application.

Request
Retrieve a User Registration for the User Id and Application Id
GET/api/user/registration/{userId}/{applicationId}
Request Parameters
applicationId
[UUID]
Required
The Id of the Application that the User is registered for.

userId
[UUID]
Required
The Id of the User whose registration is being retrieved.

Request Headers
X-FusionAuth-TenantId
[String]
Optional
The unique Id of the tenant used to scope this API request.

The tenant Id is not required on this request even when more than one tenant has been configured because the tenant can be identified based upon the request parameters or it is otherwise not required.

Specify a tenant Id on this request when you want to ensure the request is scoped to a specific tenant. The tenant Id may be provided through this header or by using a tenant locked API key to achieve the same result.

See Making an API request using a Tenant Id for additional information.

Response
The response for this API contains the User Registration.

Code Description
200 The request was successful. The response will contain a JSON body.
400 The request was invalid and/or malformed. The response will contain an Errors JSON Object with the specific errors. This status will also be returned if a paid FusionAuth license is required and is not present.
401 You did not supply a valid Authorization header. The header was omitted or your API key was not valid. The response will be empty. See Authentication.
404 The object you requested doesn't exist. The response will be empty.
500 There was an internal error. A stack trace is provided and logged in the FusionAuth log files. The response will be empty.
503 The search index is not available or encountered an exception so the request cannot be completed. The response will contain a JSON body.
Response Body
registration.applicationId
[UUID]
The Id of the Application that this registration is for.

registration.authenticationToken
[String]
The authentication token that may be used in place of the User’s password when authenticating against this application represented by this registration.

registration.cleanSpeakId
[UUID]
This Id is used by FusionAuth when the User’s username for this registration is sent to CleanSpeak to be moderated (filtered and potentially sent to the approval queue). It is the content Id of the username inside CleanSpeak.

registration.data
[Object]
An object that can hold any information about the User for this registration that should be persisted.

registration.id
[UUID]
The Id of this registration.

registration.insertInstant
[Long]
The instant that this registration was created.

registration.lastLoginInstant
[Long]
The instant that the User last logged into the Application for this registration.

registration.preferredLanguages
[Array<String>]
An array of locale strings that give, in order, the User’s preferred languages for this registration. These are important for email templates and other localizable text. See Locales.

registration.roles
[Array<String>]
The list of roles that the User has for this registration.

registration.timezone
[String]
The User’s preferred timezone for this registration. The string will be in an IANA time zone format.

registration.tokens
[Map<String,String>]
DEPRECATED
A map that contains tokens returned from identity providers.

For example, if this user has authenticated using the Facebook Identity Provider, the Facebook access token will be available in this map, keyed by name Facebook. For an OpenID Connect Identity provider, or other generic providers, if a token is stored it will be keyed by the Identity Provider unique Id.

Removed In 1.28.0
The token returned and stored from the Identity Provider is now stored in the IdP link and is retrievable using the Identity Provider Link API.

registration.username
[String]
The username of the User for this Application only.

registration.usernameStatus
[String]
The current status of the username. This is used if you are moderating usernames via CleanSpeak. The possible values are:

ACTIVE - the username is active
PENDING - the username is pending approval/moderation
REJECTED - the username was rejected during moderation
If a username has been rejected, it is still possible to allow the User to update it and have the new one moderated again.

registration.verified
[Boolean]
This value indicates if this User’s registration has been verified.

Example Response JSON
{
"registration": {
"applicationId": "10000000-0000-0002-0000-000000000001",
"data": {
"displayName": "Johnny",
"favoriteSports": [
"Football",
"Basketball"
]
},
"id": "00000000-0000-0002-0000-000000000000",
"insertInstant": 1446064706250,
"lastLoginInstant": 1456064601291,
"preferredLanguages": [
"en",
"fr"
],
"roles": [
"user",
"community_helper"
],
"timezone": "America/Chicago",
"username": "johnny123",
"usernameStatus": "ACTIVE",
"verified": true,
"verifiedInstant": 1698772159415
}
}