<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
</head>
<body>
<button>Кнопка</button>
<div style="display:none;">Форма</div>
<script>
document.querySelector('button').onclick = function(){
this.style.display = "none";
document.querySelector('div').style.display = "block";
};
</script>
</body>
</html>