PutMessage.groovy

ConfigFileAuthenticationDetailsProvider provider =  new ConfigFileAuthenticationDetailsProvider('/path/to/.oci/config', 'DEFAULT')
StreamClient client = new StreamClient(provider)
String key = 'Key'
String msg = 'Message'

PutMessagesDetails putMessageDetails = PutMessagesDetails.builder()
    .messages([
        PutMessagesDetailsEntry.builder()
            .key(key.getBytes(Charset.forName("UTF-8")))
            .value(msg.getBytes(Charset.forName("UTF-8")))
            .build()
    ])
    .build()
PutMessagesRequest putMessageRequest = PutMessagesRequest.builder()
    .streamId(this.streamId)
    .putMessagesDetails(putMessageDetails)
    .build()
client.putMessages(putMessageRequest)