code: public Blob setTemporaryHold(String bucketName, String blobName) throws StorageException {
      // [START storage_set_temporary_hold]
      // Instantiate a Google Cloud Storage client
      Storage storage = StorageOptions.getDefaultInstance().getService();
  
      // The name of a bucket, e.g. "my-bucket"
      // String bucketName = "my-bucket";
  
      // The name of a blob, e.g. "my-blob"
      // String blobName = "my-blob";
  
      BlobId blobId = BlobId.of(bucketName, blobName);
      Blob blob = storage.update(BlobInfo.newBuilder(blobId).setTemporaryHold(true).build());
  
      System.out.println("Temporary hold was set for " + blobName);
      // [END storage_set_temporary_hold]
      return blob;
    }
docs: Example of how to set a temporary hold for a blob