Bulma class: .is-multiple

    <div class="select is-multiple">
        <select multiple size="5">
            <option value="option1">Option 1</option>
            <option value="option2">Option 2</option>
            <option value="option3">Option 3</option>
            <option value="option4">Option 4</option>
            <option value="option5">Option 5</option>
        </select>
    </div>