how to login to microsoftonline.com/your.tenancy.xyz using WebRequest

login.cs

using System;
using System.Collections.Generic;
using System.Configuration;
using System.IO;
using System.IO.Compression;
using System.Linq;
using System.Net;
using System.Text;
using System.Text.RegularExpressions;
using System.Threading.Tasks;
using System.Web;

namespace Dynamics365
{
    public class MicrosoftOnlineLogin
    {
        public static CookieContainer Login(string username, string password, string application)
        {
            var wtrealm = HttpUtility.UrlEncode("spn:00000015-0000-0000-c000-000000000000");
            var wreply = HttpUtility.UrlEncode(application);
            var tenant = username.Split('@').Last().ToLower();
            var interactiveUrl = $"https://login.microsoftonline.com/{tenant}/wsfed?wa=wsignin1.0&wtrealm={wtrealm}&wreply={wreply}";
            var loginUrl = $"https://login.microsoftonline.com/{tenant}/login";

            var cookies = new CookieContainer();

            // 1. GET ms login page
            HttpWebRequest getLoginClient = (HttpWebRequest)WebRequest.Create(interactiveUrl);
            getLoginClient.CookieContainer = cookies;
            var getLoginResponse = getLoginClient.GetResponse();
            var getLoginResponseStream = new StreamReader(getLoginResponse.GetResponseStream());
            var getLoginResponseBody = getLoginResponseStream.ReadToEnd();

            // 2. POST to ms/tenant/login
            HttpWebRequest postLoginClient = (HttpWebRequest)WebRequest.Create(loginUrl);
            postLoginClient.CookieContainer = cookies;
            postLoginClient.Headers.Clear();
            postLoginClient.Headers.Add("accept-Encoding: gzip, deflate, br");
            postLoginClient.Headers.Add("Accept-Language: en-US,en;q=0.8;en-GB;q=0.6");

            // 2.b. copy cookie in to POST
            // postLoginClient.Headers.Add("Cookie", getLoginResponse.Headers["Set-Cookie"]);

            // 2.c. prepare body
            var postLoginBody = (new Dictionary<string, string>()
            {
                ["login"] = username,
                ["passwd"] = password,
                ["ctx"] = Regex.Match(getLoginResponseBody, "hidden.*ctx.*value=\"(.*)\"").Groups[1].Value,
                ["flowToken"] = Regex.Match(getLoginResponseBody, "hidden.*flowToken.*value=\"(.*)\"").Groups[1].Value,
                ["canary"] = Regex.Match(getLoginResponseBody, "hidden.*canary.*value=\"(.*)\"").Groups[1].Value,
                ["type"] = "11",
                ["LoginOptions"] = "3",
                ["NewUser"] = "1",
                ["idsbho"] = "1",
                ["PwdPad"] = "",
                ["sso"] = "",
                ["vv"] = "",
                ["uiver"] = "1"
            }).Aggregate(HttpUtility.ParseQueryString(""), (seed, current) =>
            {
                seed.Add(current.Key, current.Value);
                return seed;
            }).ToString();

            postLoginClient.Method = "POST";
            postLoginClient.ContentType = "application/x-www-form-urlencoded";
            postLoginClient.ContentLength = postLoginBody.Length;

            // 2.d. dispatch request and read response
            var postLoginRequestStream = postLoginClient.GetRequestStream();
            using (var sw = new StreamWriter(postLoginRequestStream))
            {
                sw.Write(postLoginBody);
                sw.Flush();
                sw.Close();
            }
            var postLoginResponse = postLoginClient.GetResponse();
            var postLoginResponseBody = string.Empty;
            using (var decomp = new GZipStream(postLoginResponse.GetResponseStream(), CompressionMode.Decompress))
            {
                byte[] buffer = new byte[1024];
                using (var ms = new MemoryStream())
                {
                    int nRead;
                    while ((nRead = decomp.Read(buffer, 0, buffer.Length)) > 0)
                    {
                        ms.Write(buffer, 0, nRead);
                    }
                    ms.Position = 0;
                    var tr = new StreamReader(ms);
                    postLoginResponseBody = tr.ReadToEnd();
                }
            }

            // 3. POST to app instance
            HttpWebRequest axPostClient = (HttpWebRequest)WebRequest.Create(application);
            axPostClient.CookieContainer = cookies;

            axPostClient.Headers.Add("Cache-Control: max-age=0");
            axPostClient.Headers.Add("Origin: https://login.microsoftonline.com");
            axPostClient.Headers.Add("Upgrade-Insecure-Requests: 1");
            //axPostClient.Headers.Add("User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/53.0.2767.0 Safari/537.36");
            axPostClient.ContentType = "application/x-www-form-urlencoded";
            axPostClient.Accept = "text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8";
            axPostClient.Referer = loginUrl;
            axPostClient.Headers.Add("Accept-Encoding: gzip, deflate, br");
            axPostClient.Headers.Add("Accept-Language: en-GB,en-US;q=0.8,en;q=0.6");

            var axPostBody = (new Dictionary<string, string>()
            {
                ["wa"] = "wsignin1.0",
                ["wresult"] =
                HttpUtility.HtmlDecode(Regex.Match(postLoginResponseBody, "name=\"wresult\" value=\"([^\"]*)\"").Groups[1].Value),
                ["wctx"] = "rm=0&id=passive&ru=/"
            }).Aggregate(HttpUtility.ParseQueryString(""), (seed, current) =>
            {
                seed.Add(current.Key, current.Value);
                return seed;
            }).ToString();

            axPostClient.Method = "POST";
            axPostClient.ContentLength = axPostBody.Length;

            var axPostRequestStream = axPostClient.GetRequestStream();
            using (var sw = new StreamWriter(axPostRequestStream))
            {
                sw.Write(axPostBody);
                sw.Flush();
                sw.Close();
            }
            var axPostResponse = axPostClient.GetResponse();
            var axPostResponseBody = string.Empty;
            using (var decomp = new GZipStream(axPostResponse.GetResponseStream(), CompressionMode.Decompress))
            {
                byte[] buffer = new byte[1024];
                using (var ms = new MemoryStream())
                {
                    int nRead;
                    while ((nRead = decomp.Read(buffer, 0, buffer.Length)) > 0)
                    {
                        ms.Write(buffer, 0, nRead);
                    }
                    ms.Position = 0;
                    var tr = new StreamReader(ms);
                    axPostResponseBody = tr.ReadToEnd();
                }
            }
            return cookies;
        }
    }
}

If you are trying to integrate build tools with Dynamics AX7 / Dynamics 365 and you can't get hold of your Office 365 administrator then you might need something like this!