AcquireTokenForApplication.cs

internal static async Task<string> AcquireTokenForApplication()
{
  var tenant = System.Environment.GetEnvironmentVariable("Tenant", EnvironmentVariableTarget.Process);
  var clientId = System.Environment.GetEnvironmentVariable("ClientId", EnvironmentVariableTarget.Process);
  var secret = System.Environment.GetEnvironmentVariable("Secret", EnvironmentVariableTarget.Process);
  var authorityUri = $"https://login.microsoftonline.com/{tenant}.onmicrosoft.com";
  var resourceUri = "https://manage.office.com";

  var microsoftToken = await GetTokenRetry(resourceUri, authorityUri, clientId, secret);

  return microsoftToken; 
}

private static async Task<string> GetTokenRetry(string resourceUri, string authorityUri, string clientId, string secret, int retryCount = 5, int delay = 500)
{
 ...
 AuthenticationContext authContext = new AuthenticationContext(authorityUri, false);
 ClientCredential clientCred = new ClientCredential(clientId, secret);
 var authenticationResult = await authContext.AcquireTokenAsync(resourceUri, clientCred);
 token = authenticationResult.AccessToken;
 return token;
 ...
}