Payments infrastructure your software can build on.

# 1. Open a session for one payment (page created once, in advance)
curl -X POST "$UPG_BASE_URL/api/hostedpaymentpages/sessions" \
  -H "api-key: $UPG_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "hostedPageId": "{hostedPageId}",
    "label": "Order 1042",
    "amountMode": "Locked",
    "prefilledFields": { "base_amount": "10.00" },
    "expirySeconds": 900
  }'

# 2. Ask for the payment link, redirect the payer to it
curl "$UPG_BASE_URL/api/hostedpaymentpages/sessions/{sessionId}/payment-link" \
  -H "api-key: $UPG_API_KEY"

# 3. Confirm the result from the signed webhook, not the redirect

using var http = new HttpClient { BaseAddress = new Uri(baseUrl) };
http.DefaultRequestHeaders.Add("api-key", apiKey);

var res = await http.PostAsJsonAsync("/api/hostedpaymentpages/sessions", new
{
    hostedPageId,
    label = "Order 1042",
    amountMode = "Locked",
    prefilledFields = new Dictionary<string,string> { ["base_amount"] = "10.00" },
    expirySeconds = 900
});
res.EnsureSuccessStatusCode();
// then GET .../sessions/{id}/payment-link and redirect