JSFiddle - React, Tailwind, and code Playground

HTML

<form id="Form" action="" method="post" accept-charset="ISO-8859-1" onsubmit="return checkForm(this);">
    <label>Antal biljetter:
        <select name="antal">
            <option value="">Välj ett alternativ</option>
            <option value="1">1 st</option>
            <option value="2">2 st</option>
            <option value="3">3 st</option>
            <option value="4">4 st</option>
        </select>
    </label>
    <p>
        <input type="submit" value="Skicka beställning">
</form>
<script type="text/javascript">
    function checkForm(obj) {
        if (obj.antal.selectedIndex == 0) {
            alert("Du måste välja antal biljetter.");
            return false;
        } else {
            return true;
        }
    }
</script>