Identity server identity token is always null - asp.net-core

Question

I'm using identity server with .net core application.
Current scenario: Getting token from Facebook, authenticating the user using Facebook API. And then creating a user in our database.
Everything is working fine, except that the returned token always have null identity_token.
I've tried to add id_token token to the scopes but didn't work also.
Login API:
[HttpPost("SocialLogin")]
public async Task<IActionResult> SocialMediaLogin()
{
try
{
var protocol = _httpContextAccessor.HttpContext.Request.Scheme;
var host = _httpContextAccessor.HttpContext.Request.Host.Value;
var baseUrl = $"{protocol}://{host}/";
#region Validating Headers and Parameters
StringValues jwtToken = Request.Headers["token"];
if (!jwtToken.Any())
{
return BadRequest("Each user should have a token");
}
var secretKey = _configuration["SecretKey"].ToString();
var (status, paylod) = GeneralUtilities.CheckSocialMediaTokenHeaderAndValues(jwtToken, secretKey);
if (status != ResponseStatus.Success)
{
return BadRequest("User Id or token are wrong");
}
#endregion
var clientId = _configuration["Clients:Mobile:Id"].ToString();
var secret = _configuration["Clients:Mobile:Secret"].ToString();
var consumerKey = _configuration["ConsumerKey"].ToString();
var consumerKeySecret = _configuration["ConsumerKeySecret"].ToString();
var disco = await DiscoveryClient.GetAsync(baseUrl);
if (disco.IsError) throw new Exception(disco.Error);
var tokenClient = new TokenClient(disco.TokenEndpoint, clientId, secret);
var payload = new
{
provider = paylod.Provider,
external_token = paylod.Token,
email = paylod.Email,
profilePicture = paylod.ProfilePictureUrl,
twitter_secret = paylod.TwitterSecret,
consumerKeySecret,
consumerKey,
displayName = paylod.DisplayName
};
//If user exist, we should update the profile Url of the user
var user = await _userManager.FindByEmailAsync(payload.email);
if (user != null)
{
user.ProfilePictureUrl = payload.profilePicture;
await _userManager.UpdateAsync(user);
}
var result = await tokenClient.RequestCustomGrantAsync(grantType: "external", scope: "my-api offline_access", extra: payload);
if (result.IsError) return new JsonResult(result.Json);
if (!string.IsNullOrWhiteSpace(result.AccessToken))
{
return Ok(result);
}
return new JsonResult(null);
}
catch (Exception)
{
return BadRequest();
}
}
As for the Startup.cs here's what I'm doing for Identity:
services.AddIdentity<AppUser, IdentityRole>()
.AddEntityFrameworkStores<ApplicationDbContext>()
.AddDefaultTokenProviders();
services.AddIdentityServer()
.AddOperationalStore(options =>
options.ConfigureDbContext = builder =>
builder.UseSqlServer(connectionString, sqloptions => sqloptions.MigrationsAssembly(migrationsAssembly)))
.AddConfigurationStore(options =>
options.ConfigureDbContext = builder =>
builder.UseSqlServer(connectionString, sqloptions => sqloptions.MigrationsAssembly(migrationsAssembly)))
.AddAspNetIdentity<AppUser>()
.AddDeveloperSigningCredential();
// Configure Identity
services.Configure<IdentityOptions>(options =>
{
// Password settings
options.Password.RequireDigit = true;
options.Password.RequireNonAlphanumeric = false;
options.Password.RequireUppercase = false;
options.Password.RequireLowercase = false;
});
Where does the identity_token or access_token are built?