JSFiddle - React, Tailwind, and code Playground

HTML

<form action="./" method="post">
    <input id="hidden" type="hidden" name="test">
    <button id="submit">Senden</button>
</form>

JavaScript

(function() {
    var button = document.querySelector("#submit");
    var hidden = document.querySelector("#hidden");
    button.onclick = function() {
        var c = prompt("Wie viele...");
        hidden.value = c;
    };
})();