JSFiddle - React, Tailwind, and code Playground

HTML

<select id="choix-ville">
<option data-insee="1234">Paris</option>
<option data-insee="4567">Lille</option>
<option data-insee="7890" >Lyon</option>
</select>
<p>
    <label>Code INSEE:</label>
    <input type="text" id="code-insee" />
</p>

JavaScript

function updateInseeCode(){
    var input = document.getElementById("code-insee");
    var select = document.getElementById("choix-ville");
    var selectedOption = select.options[select.selectedIndex];
    input.value = selectedOption.getAttribute("data-insee") || "";
}

updateInseeCode();
document.getElementById("choix-ville").onchange = updateInseeCode;