code: @BetaApi
    public final Operation addSignedUrlKeyBackendService(
        ProjectGlobalBackendServiceName backendService, SignedUrlKey signedUrlKeyResource) {
  
      AddSignedUrlKeyBackendServiceHttpRequest request =
          AddSignedUrlKeyBackendServiceHttpRequest.newBuilder()
              .setBackendService(backendService == null ? null : backendService.toString())
              .setSignedUrlKeyResource(signedUrlKeyResource)
              .build();
      return addSignedUrlKeyBackendService(request);
    }
docs: Adds a key for validating requests with signed URLs for this backend service.
  
  <p>Sample code:
  
  <pre><code>
  try (BackendServiceClient backendServiceClient = BackendServiceClient.create()) {
  ProjectGlobalBackendServiceName backendService = ProjectGlobalBackendServiceName.of("[PROJECT]", "[BACKEND_SERVICE]");
  SignedUrlKey signedUrlKeyResource = SignedUrlKey.newBuilder().build();
  Operation response = backendServiceClient.addSignedUrlKeyBackendService(backendService, signedUrlKeyResource);
  }
  </code></pre>
  
  @param backendService Name of the BackendService resource to which the Signed URL Key should be
  added. The name should conform to RFC1035.
  @param signedUrlKeyResource Represents a customer-supplied Signing Key used by Cloud CDN Signed
  URLs
  @throws com.google.api.gax.rpc.ApiException if the remote call fails