<input type="button" value="START" id="test" onclick="console.log('1')">
document.getElementById('test').onclick = () => console.log('2');
<input type="button" value="START" id="test" onclick="console.log('1')" />
const el = document.getElementById('test')
el.addEventListener('click', () => console.log('2'));
el.addEventListener('click', () => console.log('3'));