html

<select id="actionSelect">
  <option value="open">window.openを実行</option>
  <option value="copy">クリップボードにコピー</option>
</select>

js

document.addEventListener('DOMContentLoaded', () => {
  const select = document.getElementById('actionSelect');
  
  select.addEventListener('change', (e) => {
    if (e.target.value === 'open') {
      window.open('https://example.com', '_blank');
    } else if (e.target.value === 'copy') {
      navigator.clipboard.writeText('テストテキスト');
    }
  });
});