token.cs

public ActionResult GetTokenFromThinkTecture()
{
    var form = new FormUrlEncodedContent(
        new Dictionary<string, string>
        {
            {OAuth2Constants.GrantType, OAuth2Constants.Password},
            {OAuth2Constants.UserName, "user},
            {OAuth2Constants.Password, "password"},
            {OAuth2Constants.Scope, "https://localhost:44300/OAuth"}
        });

    var idSrvClient = new HttpClient();
    idSrvClient.DefaultRequestHeaders.Authorization =
      new BasicAuthenticationHeaderValue(
        "test_client",
        "test_secret");

    var result = idSrvClient.PostAsync(new Uri("https://idsrv/issue/oauth2/token"), form).Result;
    var tokenFromThinkTecture = new ContentResult();
    tokenFromThinkTecture.Content = result.Content.ReadAsStringAsync().Result;
    return tokenFromThinkTecture;
}

// Borrowed from https://github.com/thinktecture/Thinktecture.IdentityModel.45/blob/master/IdentityModel/Thinktecture.IdentityModel/Constants/OAuth2Constants.cs
public static class OAuth2Constants
{
public const string GrantType = "grant_type";
public const string UserName  = "username";
public const string Scope     = "scope";
public const string Assertion = "assertion";
public const string Password  = "password";
public const string Code = "code";
public const string RedirectUri = "redirect_uri";

public static class GrantTypes
{
    public const string Password          = "password";
    public const string AuthorizationCode = "authorization_code";
    public const string ClientCredentials = "client_credentials";
    public const string RefreshToken      = "refresh_token";
    public const string JWT               = "urn:ietf:params:oauth:grant-type:jwt-bearer";
    public const string Saml2             = "urn:ietf:params:oauth:grant-type:saml2-bearer";
}

public static class ResponseTypes
{
    public const string Token = "token";
    public const string Code  = "code";
}

public static class Errors
{
    public const string Error                   = "error";
    public const string InvalidRequest          = "invalid_request";
    public const string InvalidClient           = "invalid_client";
    public const string InvalidGrant            = "invalid_grant";
    public const string UnauthorizedClient      = "unauthorized_client";
    public const string UnsupportedGrantType    = "unsupported_grant_type";
    public const string UnsupportedResponseType = "unsupported_response_type";
    public const string InvalidScope            = "invalid_scope";
    public const string AccessDenied            = "access_denied";
}
}


public class BasicAuthenticationHeaderValue : AuthenticationHeaderValue
{
public BasicAuthenticationHeaderValue(string userName, string password)
    : base("Basic", EncodeCredential(userName, password))
{ }

private static string EncodeCredential(string userName, string password)
{
    Encoding encoding = Encoding.GetEncoding("iso-8859-1");
    string credential = String.Format("{0}:{1}", userName, password);

    return Convert.ToBase64String(encoding.GetBytes(credential));
}
}