Using Azure AD B2C custom policies to implement Profile Edit on Entra External ID with Native auth

ciamHelper.cs

if (method == "auth")
{
    Console.WriteLine("\n" + "Authenticating user");

    using (var httpClient = new HttpClient())
    {
        // Add Host header
        httpClient.DefaultRequestHeaders.Host = "externaltenant.ciamlogin.com";

        // Step 1: Initiate
        var initiateUrl = "https://externaltenant.ciamlogin.com/externaltenant.onmicrosoft.com/oauth2/v2.0/initiate";
        var initiateRequestBody = new Dictionary<string, string>
        {
            // TODO
            { "client_id", "your client ID" },
            { "challenge_type", "password redirect" },
            { "username", email }
        };
        var initiateContent = new FormUrlEncodedContent(initiateRequestBody);

        Console.WriteLine("\n" + "Initiate URL: " + initiateUrl);
        Console.WriteLine("\n" + "Initiate Request Body: " + JsonConvert.SerializeObject(initiateRequestBody));

        var initiateResponse = await httpClient.PostAsync(initiateUrl, initiateContent);
        var initiateResponseContent = await initiateResponse.Content.ReadAsStringAsync();

        Console.WriteLine("\n" + "Initiate Response Status Code: " + initiateResponse.StatusCode);
        Console.WriteLine("\n" + "Initiate Response Content: " + initiateResponseContent);

        if (!initiateResponse.IsSuccessStatusCode)
        {
            return new ConflictObjectResult(new B2CResponseModel($"Failed to initiate authentication. Response: {initiateResponseContent}", HttpStatusCode.Conflict));
        }

        var initiateJson = JsonConvert.DeserializeObject<JObject>(initiateResponseContent);
        var continuationToken = initiateJson["continuation_token"].ToString();

        // Step 2: Challenge
        var challengeUrl = "https://externaltenant.ciamlogin.com/externaltenant.onmicrosoft.com/oauth2/v2.0/challenge";
        var challengeRequestBody = new Dictionary<string, string>
        {
            // TODO
            { "client_id", "your client ID" },
            { "challenge_type", "password redirect" },
            { "continuation_token", continuationToken }
        };
        var challengeContent = new FormUrlEncodedContent(challengeRequestBody);

        var challengeResponse = await httpClient.PostAsync(challengeUrl, challengeContent);
        if (!challengeResponse.IsSuccessStatusCode)
        {
            return new ConflictObjectResult(new B2CResponseModel($"Failed to complete challenge.", HttpStatusCode.Conflict));
        }

        var challengeResponseContent = await challengeResponse.Content.ReadAsStringAsync();
        var challengeJson = JsonConvert.DeserializeObject<JObject>(challengeResponseContent);
        continuationToken = challengeJson["continuation_token"].ToString();

        // Step 3: Token
        var tokenUrl = "https://externaltenant.ciamlogin.com/externaltenant.onmicrosoft.com/oauth2/v2.0/token";
        var tokenRequestBody = new Dictionary<string, string>
        {
            // TODO
            { "client_id", "your client ID" },
            { "password", password },
            { "continuation_token", continuationToken },
            { "grant_type", "password" },
            { "scope", "openid offline_access" }
        };
        var tokenContent = new FormUrlEncodedContent(tokenRequestBody);

        var tokenResponse = await httpClient.PostAsync(tokenUrl, tokenContent);
        
        if (!tokenResponse.IsSuccessStatusCode)
        {
            return new ConflictObjectResult(new B2CResponseModel($"Failed to obtain token.", HttpStatusCode.Conflict));
        }

        // Read the response content
        var responseContent = await tokenResponse.Content.ReadAsStringAsync();
        var jsonObj = JsonConvert.DeserializeObject(responseContent);

        Console.WriteLine("\n" + "jsonObject " + jsonObj.ToString());

        return new OkObjectResult(jsonObj);                    
    }
}

https://medium.com/the-new-control-plane/using-azure-ad-b2c-custom-policies-to-implement-profile-edit-on-entra-external-id-with-native-auth-1a2587bda05d