JSFiddle - React, Tailwind, and code Playground

HTML

<select id="VatExpense">
    <option value="" disabled selected>Expense Type</option>
    <option value="0">Telephone</option>
    <option value="1">Public Transport & Taxis</option>
    <option value="0">Computer Consumables</option>
    <option value="0">Subsistence</option>
    <option value="1">Overseas Travel</option>
</select>
<input type="text" id="vat" placeholder="Rate">

JavaScript

$("#VatExpense").on("change", function() {
    var vatExpense = $("#VatExpense option:selected").val();
    var vatPlaceholder = "Rate";
    if (vatExpense == 0) {
        vatPlaceholder = "Standard rate 20%";
    } else if (vatExpense == 1) {
        vatPlaceholder = "Zero Rate 0%";
    }
    $("#vat").attr("placeholder", vatPlaceholder);
});