var messageIndex = 1;

var messageRate = 1500;
function sendMessage(){
    print('http://www.youtube.com/embed/dP15zlyra3c?html5=1&autoplay=1&start='+20);
Messages.sendMessage('messageTest',messageIndex);
messageIndex++;
}

function cleanup(){
    Script.clearInterval(messageInterval);
}

var messageInterval = Script.setInterval(function(){
    sendMessage();
},messageRate);

Script.scriptEnding.connect(cleanup);