def METHOD_NAME(event):
    channel = event['channel']
    print(channel)
    dynamodb_client = boto3.client('dynamodb')
    dynamodb_client.put_item(
        TableName = subscriptions_table_name,
        Item = {
            "id": {
                "S": event['user'],
            },
            "channel": {
                "S": event['channel']
            }
        }
    )
    text = "Hi <@"+event['user']+">. You've been subscribed to @ mentions. Send me a message containing 'unsubscribe' to stop receiving those notifications."
    print(text)
    send_channel_message(event['channel'], text)