How to get access token from HttpContext in .Net core 2.0

In Controller, the token can be retrieved by reading Request.Headers dictionary:

 var accessToken = Request.Headers["Authorization"];

At other classes where HttpContext is not available, there token can be retrieved using HttpContextAccessor after injecting into services collection ( A little change from Azharuddin answer)

Register the service instance in Startup method like

public void ConfigureServices(IServiceCollection services)
{

 services.AddSingleton<IHttpContextAccessor, HttpContextAccessor>();
 ...
}

And inject the dependency in your controller like

private IHttpContextAccessor _httpContextAccessor;
public ClientController(IHttpContextAccessor httpContextAccessor)
{
     _httpContextAccessor = httpContextAccessor;
}

And retrieve the access token in your action like

[Authorize]
public async Task<IActionResult> ClientUpdate(ClientModel client)
{
    var accessToken = _httpContextAccessor.HttpContext.Request.Headers["Authorization"];

    ..........//Some other code
    return View();
}

.Net core 2.1 to access JWT bearer token

var accessToken = Request.Headers[HeaderNames.Authorization];

It ended up being a configuration issue. There needs to be a link between AddAuthentication and AddOpenIdConnect in order for it to read the cookie into the headers.

services.TryAddSingleton<IHttpContextAccessor, HttpContextAccessor>();

services.AddAuthentication(options =>
            {
                options.DefaultScheme = "Cookies";
                options.DefaultChallengeScheme = "oidc";
            })
            .AddCookie("Cookies")
            .AddOpenIdConnect("oidc", options =>
            {
                options.SignInScheme = "Cookies";

                options.Authority = "http://localhost:5000";
                options.RequireHttpsMetadata = false;

                options.ClientId = "testclient";
                options.ClientSecret = "secret";
                options.ResponseType = "code id_token";
                options.SaveTokens = true;
                options.GetClaimsFromUserInfoEndpoint = true;

                options.Scope.Add("testapi");
                options.Scope.Add("offline_access");
            });

Controller

    [Authorize]
    public async Task<IActionResult> Index()
    {
        var accessToken = await HttpContext.GetTokenAsync("access_token");
        return View();
    }

Access token is now populated.

Note: I ended up digging it out of this project Startup.cs

if you want the pure token this can help you in .net core 3.1

var _bearer_token = Request.Headers[HeaderNames.Authorization].ToString().Replace("Bearer ", "");

and remember you need to add this using

using Microsoft.Net.Http.Headers;