JSFiddle - React, Tailwind, and code Playground

HTML

<select id="multiSelect" size="5" multiple="multiple">
    <option value="o1">Option 1</option>
    <option value="o2">Option 2</option>
    <option value="o3">Option 3</option>
    <option value="o4">Option 4</option>
    <option value="o5">Option 5</option>
    <option value="o6">Option 6</option>
</select>
<br />
<button id="btnGetValues" onclick="GetValues()">Get Values</button>
<br />
<br />
<br />
<input class="text" id="quantite" type="text" name="quantité" value="o7" />

JavaScript

function GetValues() {
    var myList = document.getElementById("multiSelect");
    var option = document.createElement('option');
    option.value = document.getElementById('quantite').value;
    option.innerText = 'New Option';
    myList.appendChild(option);
}