SolomonRei,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
.mfp-hide{
display: none;
}
.white-popup {
position: relative;
background-color: #FFFFFF;
padding: 20px;
width: auto;
max-width: 500px;
margin: 20px auto;
}
</style>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.4/jquery.min.js"></script>
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/magnific-popup.js/1.1.0/magnific-popup.css">
<script src="https://cdnjs.cloudflare.com/ajax/libs/magnific-popup.js/1.1.0/jquery.magnific-popup.min.js"></script>
<script>
jQuery(document).ready(function($) {
var tm;
$('.popup-with-form-allabout').click(function(event){
event.preventDefault();
window.clearTimeout(tm);
tm = window.setTimeout(function() {
$.magnificPopup.open({
items : {src : '#form-popup', type: 'inline'}
});
}, 5000)
});
});
</script>
</head>
<body>
<a href="#form-popup" class = "popup-with-form-allabout" style = "text-decoration: none;"><div class = "mainButton">Написать нам</div></a>
<div id="form-popup" class="white-popup mfp-hide" >
test
</div>
</body>
</html>