Use Azure AD Graph to update values on the `AdditionalValues` dictionary for a user - azure-active-directory

Question

How do I use Azure AD Graph to update values on the AdditionalValues dictionary for a user? The test below returns 400 Bad Response.
Background:
The rest of my application uses MSGraph. However, since a federated user can not be updated using MSGraph I am searching for alternatives before I ditch every implementation and version of Graph and implement my own database.
This issue is similar to this one however in my case I am trying to update the AdditionalData property.
Documentation
[TestMethod]
public async Task UpdateUserUsingAzureADGraphAPI()
{
string userID = "a880b5ac-d3cc-4e7c-89a1-123b1bd3bdc5"; // A federated user
// Get the user make sure IsAdmin is false.
User user = (await graphService.FindUser(userID)).First();
Assert.IsNotNull(user);
if (user.AdditionalData == null)
{
user.AdditionalData = new Dictionary<string, object>();
}
else
{
user.AdditionalData.TryGetValue(UserAttributes.IsCorporateAdmin, out object o);
Assert.IsNotNull(o);
Assert.IsFalse(Convert.ToBoolean(o));
}
string tenant_id = "me.onmicrosoft.com";
string resource_path = "users/" + userID;
string api_version = "1.6";
string apiUrl = $"https://graph.windows.net/{tenant_id}/{resource_path}?{api_version}";
// Set the field on the extended attribute
user.AdditionalData.TryAdd(UserAttributes.IsCorporateAdmin, true);
// Serialize the dictionary and put it in the content of the request
string content = JsonConvert.SerializeObject(user.AdditionalData);
string additionalData = "{\"AdditionalData\"" + ":" + $"[{content}]" + "}";
//additionalData: {"AdditionalData":[{"extension_myID_IsCorporateAdmin":true}]}
HttpClient httpClient = new HttpClient();
HttpRequestMessage request = new HttpRequestMessage
{
Method = HttpMethod.Patch,
RequestUri = new Uri(apiUrl),
Content = new StringContent(additionalData, Encoding.UTF8, "application/json")
};
var response = await httpClient.SendAsync(request); // 400 Bad Request
}

Answer 1

Make sure that the Request URL looks like: https://graph.windows.net/{tenant}/users/{user_id}?api-version=1.6. You need to change the api_version to "api-version=1.6".
You cannot directly add extensions in AdditionalData and it will return the error(400).
Follow the steps to register an extension then write an extension value to user.
Register an extension:
POST https://graph.windows.net/{tenant}/applications/<applicationObjectId>/extensionProperties?api-version=1.6
{
"name": "<extensionPropertyName like 'extension_myID_IsCorporateAdmin>'",
"dataType": "<String or Binary>",
"targetObjects": [
"User"
]
}
Write an extension value:
PATCH https://graph.windows.net/{tenant}/users/{user-id}?api-version=1.6
{
"<extensionPropertyName>": <value>
}