How can I pass parameters in javascript fetch call - asp.net

Question

I'm integrating Stripe payments in an aspx site (VB, .NET 4.7.2).
I took Javascripts from Stripe samples.
I try to call this method of the backend:
<WebMethod()>
Public Shared Function CreateStripePaymentIntent(ByVal aPrice As String) As PaymentIntent
...
End Function
from a Javascript in the page:
(here the parameter is a fixed value, to simplify)
fetch("PayOrder.aspx/CreateStripePaymentIntent?aPrice=111", {
method: "POST",
headers: {
'Accept': 'application/json',
"Content-Type": "application/json"
}
body: JSON.stringify(purchase)
})
If I remove the parameter all works well (but is useless).
If I try to pass the parameter I always have error 500:
POST http://localhost:14987/PayOrder.aspx/CreateStripePaymentIntent?aPrice=111
Status 500
Internal Server Error
Version HTTP/1.1
Transferred 0,98 kB (660 B size)
Referrer Policy strict-origin-when-cross-origin
Message "No value for parameter 'aPrice'." (my translation from italian to english!)
Someone can tell me WHY this parameter does not reach the backend?

Answer 1

The values are sent in the request body, in the format that the content type specifies.
Usually the content type is application/x-www-form-urlencoded, so the request body uses the same format as the query string:
parameter=value&also=another
When you use a file upload in the form, you use the multipart/form-data encoding instead, which has a different format. It's more complicated, but you usually don't need to care what it looks like, so I won't show an example, but it can be good to know that it exists.
for Json content type is application/json
fetch("PayOrder.aspx/CreateStripePaymentIntent", {
method: 'POST',
headers: {
'Accept': 'application/json',
'Content-Type': 'application/json'
},
body: JSON.stringify({ aPrice: 111 })
})