formstack: add 1 captured sample

providers/formstack/README.md

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+# Formstack
+
+Formstack Forms' WebHook submit action ("Send Data to an External URL"). It fires on
+exactly one thing, a form submission, and sends no event type in any header or body field.
+That is why `topic_identifier` is `null`.
+
+`submission` is the name of the file, not a value Formstack sends. Don't match on it, and
+don't treat it as Formstack event vocabulary. Route Formstack deliveries on the `FormID`
+body field.
+
+## The sample
+
+`latest/submission.json` is a real delivery, captured on 2026-09-25 from a test form with
+no answer fields. It went through a Hookdeck source, and the `x-hookdeck-*` headers were
+removed. Everything else is as received, including the signature.
+
+- The body arrived as `application/x-www-form-urlencoded`, which is Formstack's default.
+  The exact wire bytes were `FormID=6606394&UniqueID=1500878955&HandshakeKey=test`.
+- `x-fs-signature` is HMAC-SHA256 of those bytes, as lowercase hex prefixed with
+  `sha256=`. The WebHook's HMAC Key was the throwaway value `test1`, so the signature can
+  be recomputed:
+
+  ```bash
+  printf '%s' 'FormID=6606394&UniqueID=1500878955&HandshakeKey=test' | openssl dgst -sha256 -hmac test1 -r
+  ```
+
+- `HandshakeKey` carries the WebHook's Shared Secret, here the throwaway `test`. This capture
+  had a Shared Secret set, so it doesn't show what a WebHook without one sends.
+
+A mock send re-encodes the body, and the bytes may not match the original. The
+signature is only valid against the exact wire bytes above.
+
+Formstack Documents (formerly WebMerge) is a different product with a different webhook
+and is not covered here.

providers/formstack/index.json

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+{
+  "label": "Formstack",
+  "configs": {
+    "latest_version": "latest",
+    "topic_identifier": null
+  },
+  "provenance": {
+    "latest": {
+      "sourced_via": "capture",
+      "sourced_on": "2026-09-25"
+    }
+  }
+}

providers/formstack/latest/submission.json

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+{
+  "headers": {
+    "content-length": "52",
+    "content-type": "application/x-www-form-urlencoded; charset=utf-8",
+    "tracestate": "dd=p:60d345bc96c3c585;",
+    "user-agent": "FormstackWebhook/1.0 (Form 6606394)",
+    "x-datadog-parent-id": "6976996924012610949",
+    "x-datadog-sampling-priority": "0",
+    "x-datadog-tags": "_dd.p.tid=6ab674fb00000000",
+    "x-datadog-trace-id": "16234198080647658916",
+    "x-fs-signature": "sha256=30dff7f180b6d69eab397a5d51719474df490b730514c253e8b5832d3b51b970"
+  },
+  "body": {
+    "FormID": "6606394",
+    "UniqueID": "1500878955",
+    "HandshakeKey": "test"
+  },
+  "topic": "submission"
+}