Kod formy:
<form method="get" action="/sberbank">
<input name="orderNumber" type="number" />
<input name="amount" type="number" />
<button type="submit">Donate</button>
</form>

Kod stranicy /sberbank na vashem sajte:
<?php
$price = $_GET["amount"]*100;
$info = file_get_contents("https://securepayments.sberbank.kz/payment/rest/register.do?orderNumber=".$_GET["orderNumber"]."&token=vash_token&returnUrl=adres_stranicy_uspekha&amount=".$price."&currency=398");
$info = json_decode($info, true);

header('Location: ' . $info['formUrl']);
die();
?>