Try it Editor

<!DOCTYPE html>
<html>
<body>
    <form name="frm-pin" method="post" action="pin-index-a.php">
        <input type="hidden" name="mode" value="PinRequest" />
        <label class="w3-text-green"><b>Choose price</b></label>
        <select name="tot_pin_requested" onchange="calculateAmount(this.value)" required>
		<option value="" disabled selected>Choose your option</option>
		<option value="1000">1000</option>
		<option value="2000">2000</option>
		<option value="3000">3000</option>
		<option value="4000">4000</option>
		<option value="5000">5000</option>
</select>
        <label><b>Total Amount after tax</b></label>
        <input class="w3-input w3-border" name="tot_amount" id="tot_amount" type="text" readonly>
        <script>
            function calculateAmount(val) {
                var price = val * 1;
                //display the result
                var tot_price = price + (price * 0.18);

                var divobj = document.getElementById('tot_amount');
                divobj.value = tot_price;
            }
        </script>
        <body>
</html>