<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>Pay</title>
</head>
<body>
<?php
$suma=$_POST['suma'];
<form method="GET" action="https://api.cryptonator.com/api/merchant/v1/startpayment">
<input type="hidden" name="merchant_id" value="586ddbe0fd0fba9f50f1600ea68001f9">
<input type="hidden" name="item_name" value="Оплата заказа">
<input type="hidden" name="invoice_currency" value="usd">
The Amount <input type="number" min="5" max="100000" name="suma" /><br />
<input type="hidden" name="invoice_amount" value="$suma" data-type="number">
<input type="hidden" name="language" value="ru">
<input type="submit" value="Оплатить криптовалютой">
</form>
?>
</form>
</body>
</html>