Michi

  $( ".knock" ).click(function() {
    $( ".msg" ).addClass( "sayhello" );
});

$( "body" ).on('click', '.sayhello', function() {
    alert("Hello!");
});

div { cursor: pointer; }
.msg { display: none; }
.msg.sayhello { display: block; }

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div class="knock">Knock</div>
<div class="msg">Say hello</div>