OAuth implementation for Azure Active Directory Authentication of an App Services

AzureAppServiceAuthenticator.cs

public class AzureAppServiceAuthenticator
{
    // Found in the Azure Portal > Azure Active Directory > App Registration (matching Function app name) > Overview > Directory (tenant) ID
    protected internal const string AzureActiveDirectoryTenantId = "abcd1234-abcd-1234-abcd-1234abcd123";
    private static readonly HttpClient HttpClient = new HttpClient();

    public async Task<string> GetBearerToken()
    {
        var clientId = GetClientId();
        var clientSecret = GetClientSecret();
        var content = new FormUrlEncodedContent(new[]
        {
            new KeyValuePair<string, string>("client_id", clientId),
            new KeyValuePair<string, string>("client_secret", clientSecret),
            new KeyValuePair<string, string>("grant_type", "client_credentials"),
            new KeyValuePair<string, string>("resource", clientId)
        });
        var response = await HttpClient.PostAsync($"https://login.microsoftonline.com/{AzureActiveDirectoryTenantId}/oauth2/token", content);
        if (response.IsSuccessStatusCode)
        {
            var authResponse = await response.Content.ReadAsAsync<AuthResponse>();
            return authResponse.access_token;
        }
        var message = $"Unable to acquire bearer token. Response Status: {response.StatusCode}.";
        throw new Exception(message);
    }

    private static string GetClientId()
    {
        // Found in the Azure Portal > Azure Active Directory > App Registration (matching Function app name) > Overview > Application (client) ID
        return ConfigurationManager.AppSettings["MyClientId"];
    }

    private static string GetClientSecret()
    {
        // Secret created in the Azure Portal > Azure Active Directory > App Registration (matching Function app name) > Certificates & Secrets > new secret
        throw new NotImplementedException("return from keyvault after creating");
    }

    public class AuthResponse
    {
        public string token_type { get; set; }
        public string expires_in { get; set; }
        public string ext_expires_in { get; set; }
        public string expires_on { get; set; }
        public string not_before { get; set; }
        public string resource { get; set; }
        public string access_token { get; set; }
    }
}
// More information available at https://andypalmer.dev/posts/azure-active-directory-app-service/