Untitled

<!DOCTYPE HTML>
<html>
<head>
    <meta charset="utf-8">
    <title>Тег IMG</title>
</head>
<body>
<div id="elem"></div>
<button class="btn">Кликни сюда</button>
<script>
    const elem = document.querySelector('#elem');
    const button = document.querySelector('.btn');

    button.addEventListener('click', () => {
        elem.classList.add('num1');
    });
</script>
</body>
</html>