CODE EXAMPLE FOR HTML

select required

<label for="cars">Choose a car:</label>

<select name="cars" id="cars" required>
  <option value="">None</option>
  <option value="volvo">Volvo</option>
  <option value="saab">Saab</option>
  <option value="mercedes">Mercedes</option>
  <option value="audi">Audi</option>
</select>