Idea

Reference:

https://community.playfab.com/hc/en-us/community/posts/209071588-AddUsernamePassword-email-already-exists-what-?flash_digest=f401cb7eac03972d95028c67e06e275158f85d03#community_comment_212187378

Use Cases:

1. Started signup process with only email and password? This is your solution to add a username -- you currently cannot use AddUsernamePassword because it will return "EMAIL ALREADY IN USE"
   
   
2. Started with an email list and want to manually add people, then request the user for a username and password? This is the solution.
   
   
3. Someone signed up via ANYTHING other than using an email, then want to add a username? This is your solution.

// Sample Request
POST https://{ {TitleID}}.playfabapi.com/Client/UpdateUserInfo
Content-Type: application/json;
X-Authentication: <user_session_ticket_value>{
    "Username": "theuser", // Optional
    "Email": "me@here.com", // Optional
    "Password": "thepassword" // Optional
}

// Sample Response
HTTP/1.1 200 OK
Content-Type: application/json; charset=utf-8
X-Authentication: <user_session_ticket_value>{
    "code": 200,
    "status": "OK",
    "data": {
        "DataVersion": 3
    }
}