如何转换原始表单无法添加标题参数问题？

When I read the WHMCS payment gateway demo:

there gives the form for submit:

$htmlOutput = '<form method="post" action="' . $url . '">';
foreach ($postfields as $k => $v) {
    $htmlOutput .= '<input type="hidden" name="' . $k . '" value="' . urlencode($v) . '" />';
}
$htmlOutput .= '<input type="submit" value="' . $langPayNow . '" />';
$htmlOutput .= '</form>';

But I have a question, in my scenario, my payment gateway there is a header params like this:

curl https://paypal.com/v1.3/transactions/qrcode \
-H "Authorization: Bearer <TOKEN>" \
-d amount=100 \
-d currency="USD" \
-d reference="demo" \
-d ipn_url="http://paypal.com/ipn"

the origin form can not add request header in the form request. in there how can I solve this problem?

I don't know whether this need context, but how ?