Solved it myself...
after the normal message.prompt code:-
$.messager.prompt("enter a number","number",function(r)
{
if(r)
{
// your code
}
});
Add this:-
$('.messager-input').bind('keypress', function(e) {
if(e.keyCode==13)
{
$('body div.messager-body>div.messager-button').children('a.l-btn:first-child').click();
}
}
);