Tryit Editor v1.8

Source Code:

<!DOCTYPE html>
<html>
<body>

<form action="form_action.asp">
<select name="cars" multiple>
  <option value="volvo">Volvo</option>
  <option value="saab">Saab</option>
  <option value="opel">Opel</option>
  <option value="audi">Audi</option>
</select>
<input type="submit">
</form>

<p>Hold down the Ctrl (windows) / Command (Mac) button to select multiple options.</p>

</body>
</html>

Result:

W3Schools.com - Try it yourself

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