If it's not a problem for you to send the original message to the server, i.e. you problem is only that the server returns the original message after you've send it to it, then you can call the encryptTo API first and then provide the result as encryptedMessageData and encryptedMessageNonce parameters and not use the message and messageToEncrypt parameters. You won't get the original message in response, e.g.
If your problem is also that the original message is sent to the server, then you need to calculate locally the data and nonce and again provide them as encryptedMessageData and encryptedMessageNonce