From Solution Explorer, navigate to the /Controllers/ folder; then, open the TokenController.cs file and add the following action method:

[HttpPost("Facebook")]
public async Task<IActionResult> Facebook([FromBody]ExternalLoginRequestViewModel model)
{
    try
    {
        var fbAPI_url = "https://graph.facebook.com/v2.10/";
        var fbAPI_queryString = String.Format(
            "me?scope=email&access_token={0}&fields=id,name,email", 
            model.access_token);
        string result = null;

        // fetch the user info from Facebook Graph v2.10
        using (var c = new HttpClient())
        {
            c.BaseAddress = new Uri(fbAPI_url);
            var response = await c
                .GetAsync(fbAPI_queryString);
            if (response.IsSuccessStatusCode)
            {
                result = await response.Content.ReadAsStringAsync();
            }
            else throw new Exception("Authentication error");
        };

        // load the resulting Json into a dictionary
        var epInfo = JsonConvert.DeserializeObject<Dictionary<string, 
                                                 string>>(result);
        var info = new UserLoginInfo("facebook", epInfo["id"], 
                                                         "Facebook");

        // Check if this user already registered himself with this 
                                            external provider before
        var user = await UserManager.FindByLoginAsync(
            info.LoginProvider, info.ProviderKey);
        if (user == null)
        {
            // If we reach this point, it means that this user never 
                                                  tried to logged in
            // using this external provider. However, it could have 
                                                used other providers 
            // and /or have a local account. 
            // We can find out if that's the case by looking for his e-
                                                         mail address.

            // Lookup if there's an username with this e-mail address 
                                                          in the Db
            user = await UserManager.FindByEmailAsync(epInfo["email"]);
            if (user == null)
            {
                // No user has been found: register a new user using 
                                                           the info 
                // retrieved from the provider
                DateTime now = DateTime.Now;
                var username = String.Format("FB{0}{1}",
                        epInfo["id"],
                        Guid.NewGuid().ToString("N"));
                user = new ApplicationUser()
                {
                    SecurityStamp = Guid.NewGuid().ToString(),
                    // ensure the user will have an unique username
                    UserName = username,
                    Email = epInfo["email"],
                    DisplayName = epInfo["name"],
                    CreatedDate = now,
                    LastModifiedDate = now
                };

                // Add the user to the Db with a random password
                await UserManager.CreateAsync(user,
                    DataHelper.GenerateRandomPassword());

                // Assign the user to the 'RegisteredUser' role.
                await UserManager.AddToRoleAsync(user, 
                                           "RegisteredUser");

                // Remove Lockout and E-Mail confirmation
                user.EmailConfirmed = true;
                user.LockoutEnabled = false;

                // Persist everything into the Db
                DbContext.SaveChanges();
            }
            // Register this external provider to the user
            var ir = await UserManager.AddLoginAsync(user, info);
            if (ir.Succeeded)
            {
                // Persist everything into the Db
                DbContext.SaveChanges();
            }
            else throw new Exception("Authentication error");
        }

        // create the refresh token
        var rt = CreateRefreshToken(model.client_id, user.Id);

        // add the new refresh token to the DB
        DbContext.Tokens.Add(rt);
        DbContext.SaveChanges();

        // create & return the access token
        var t = CreateAccessToken(user.Id, rt.Value);
        return Json(t);
    }
    catch (Exception ex)
    {
        // return a HTTP Status 400 (Bad Request) to the client
        return BadRequest(new { Error = ex.Message });
    }
}

Don't forget to add the following required reference at the start of the file:

using System.Net.Http;

The included comments should explain it all; however, it can't hurt to briefly summarize what we did:

• We use the OAuth2 access token--which we plan to receive from our Angular app--to request the user ID, name, and email address from the Facebook API.
• Once we retrieve the information, we use the user email address to check whether the user already exists in our identity data model or not. If it doesn't, we create a new user, register the external provider, and perform the login; otherwise, we just register the external provider and perform the login.

As we might note, when we create a new user, we're forced to create a "unique" username, as it's a required field; the method we used--the "FB" string prefix + the facebook unique ID + a random Guid without the dashes--will ensure its uniqueness among the database. Other than that, there are no surprises--the code lines that generate the access/refresh JWT tokens and return the TokenResponseViewModel are nothing new.

However, there's a single line--the highlighted one--that will raise an exception; there's no DataHelper.GenerateRandomPassword() method out there. As a matter of fact, there is no DataHelper class as well! That's definitely true; we still need to add it.