If everything goes as planned, you should have successfully stored the hello world message into bitcoin's testnet blockchain.

The following screenshot illustrates what we have done so far. We consumed an input (from a previous transaction), then created a transaction with two outputs; the first being an OP_RETURN transaction carrying our message along, the other one transferring one bitcoin (BTC):

Isn't it just fascinating? You can use a block explorer such as https://live.blockcypher.com/btc-testnet/tx/<txid> to inspect the transaction with the printed transaction hash (txid), and to retrieve your stored message.

It would be more exciting to retry the same operation using the mainnet (the original and main network for bitcoin), but then you would be dealing with real, expensive bitcoins.