<!DOCTYPE html>
<html>
<head>
<meta http-equiv="content-type" content="text/html; charset=utf-8" />
<title>ГГ</title>
</head>
<body>
<input type="submit" value="Go" class="button">
<script>
var button = document.querySelector('input[type="submit"]');
button.onclick = alert('Кнопка нажата');
button.onclick();
</script>
</body>
</html>