Telcell Docs

function getTelcellSecurityCode ($shop_key, $issuer, $currency, $price, $product, $issuer_id, $valid_days) {
    return hash('md5', $shop_key . $issuer . $currency . $price . $product . $issuer_id . $valid_days);
}

<form target="_blank" action="https://telcellmoney.am/invoices" method="POST">
    <input type="hidden" name="issuer" value="test@tv4ds51.test">
    <input type="hidden" name="action" value="PostInvoice">
    <input type="hidden" name="currency" value="֏">
    <input type="hidden" name="price" value="200">
    <input type="hidden" name="product" value="0KLQvtCy0LDRgA==">
    <input type="hidden" name="issuer_id" value="MTU5MTY1NDY3Mw==">
    <input type="hidden" name="valid_days" value="10">
    <input type="hidden" name="lang" value="ru">
    <input type="hidden" name="security_code" value="50253a7fe62f22a38ff452f4a66b6f51">
    <input type="submit" value="Оплатить">
</form>