Server-side google authorization

What you are looking for is a service account. Service accounts are like dummy users. Using gsuite your gsuite admin can setup up domain wide delegation to the service account allowing it to access the calendar on the domain. Once the service account has been granted access you wont need any authorization. Its all automated server side.

public static class ServiceAccountExample
    {

        /// <summary>
        /// Authenticating to Google using a Service account
        /// Documentation: https://developers.google.com/accounts/docs/OAuth2#serviceaccount
        /// </summary>
        /// <param name="serviceAccountEmail">From Google Developer console https://console.developers.google.com</param>
        /// <param name="serviceAccountCredentialFilePath">Location of the .p12 or Json Service account key file downloaded from Google Developer console https://console.developers.google.com</param>
        /// <returns>AnalyticsService used to make requests against the Analytics API</returns>
        public static CalendarService AuthenticateServiceAccount(string serviceAccountEmail, string serviceAccountCredentialFilePath, string[] scopes)
        {
            try
            {
                if (string.IsNullOrEmpty(serviceAccountCredentialFilePath))
                    throw new Exception("Path to the service account credentials file is required.");
                if (!File.Exists(serviceAccountCredentialFilePath))
                    throw new Exception("The service account credentials file does not exist at: " + serviceAccountCredentialFilePath);
                if (string.IsNullOrEmpty(serviceAccountEmail))
                    throw new Exception("ServiceAccountEmail is required.");                

                // For Json file
                if (Path.GetExtension(serviceAccountCredentialFilePath).ToLower() == ".json")
                {
                    GoogleCredential credential;
                    using (var stream = new FileStream(serviceAccountCredentialFilePath, FileMode.Open, FileAccess.Read))
                    {
                        credential = GoogleCredential.FromStream(stream)
                             .CreateScoped(scopes);
                    }

                    // Create the  Analytics service.
                    return new CalendarService(new BaseClientService.Initializer()
                    {
                        HttpClientInitializer = credential,
                        ApplicationName = "Calendar Service account Authentication Sample",
                    });
                }
                else if (Path.GetExtension(serviceAccountCredentialFilePath).ToLower() == ".p12")
                {   // If its a P12 file

                    var certificate = new X509Certificate2(serviceAccountCredentialFilePath, "notasecret", X509KeyStorageFlags.MachineKeySet | X509KeyStorageFlags.Exportable);
                    var credential = new ServiceAccountCredential(new ServiceAccountCredential.Initializer(serviceAccountEmail)
                    {
                        Scopes = scopes
                    }.FromCertificate(certificate));

                    // Create the  Calendar service.
                    return new CalendarService(new BaseClientService.Initializer()
                    {
                        HttpClientInitializer = credential,
                        ApplicationName = "Calendar Authentication Sample",
                    });
                }
                else
                {
                    throw new Exception("Unsupported Service accounts credentials.");
                }

            }
            catch (Exception ex)
            {                
                throw new Exception("CreateServiceAccountCalendarFailed", ex);
            }
        }
    }

Code ripped from ServiceAccount.cs