fix: allow string form_data in CustomWebhookNotificationBlock

src/prefect/blocks/notifications.py

@@ -844,11 +844,12 @@ class CustomWebhookNotificationBlock(NotificationBlock):
             ' "{{tokenFromSecrets}}"}'
         ],
     )
-    form_data: Optional[dict[str, str]] = Field(
+    form_data: Optional[dict[str, str] | str] = Field(
         default=None,
         title="Form Data",
         description=(
-            "Send form data as payload. Should not be used together with _JSON Data_."
+            "Send form data as payload. Should not be used together with _JSON Data_. "
+            "Can be a dictionary for form-encoded data or a string for raw body content."
         ),
         examples=[
             '{"text": "{{subject}}\\n{{body}}", "title": "{{name}}", "token":'
@@ -882,13 +883,18 @@ def _build_request_args(self, body: str, subject: str | None) -> dict[str, Any]:
                 "name": self.name,
             }
         )
+        # httpx uses 'data' for form-encoded dicts, 'content' for raw string/bytes
+        if isinstance(self.form_data, str):
+            data_key = "content"
+        else:
+            data_key = "data"
         # do substution
         return apply_values(
             {
                 "method": self.method,
                 "url": self.url,
                 "params": self.params,
-                "data": self.form_data,
+                data_key: self.form_data,
                 "json": self.json_data,
                 "headers": self.headers,
                 "cookies": self.cookies,

tests/blocks/test_notifications.py

@@ -963,6 +963,30 @@ def test_provide_both_data_and_json_raises_validation_error(self):
                 secrets={"token": "someSecretToken"},
             )
 
+    async def test_string_form_data(self):
+        """Test that form_data accepts a string for raw body content.
+
+        This enables forwarding pre-constructed JSON from automation bodies.
+        See: https://github.com/PrefectHQ/prefect/issues/19949
+        """
+        with respx.mock(using="httpx") as xmock:
+            xmock.post("https://example.com/")
+
+            custom_block = CustomWebhookNotificationBlock(
+                name="test name",
+                url="https://example.com/",
+                form_data="{{body}}",
+                headers={"Content-Type": "application/json"},
+            )
+            # Simulate automation passing JSON as the body
+            await custom_block.notify(
+                '{"flow_name": "my-flow", "state": "Failed"}', "subject"
+            )
+
+            last_req = xmock.calls.last.request
+            assert last_req.content == b'{"flow_name": "my-flow", "state": "Failed"}'
+            assert last_req.headers["Content-Type"] == "application/json"
+
 
 class TestSendgridEmail:
     URL_PARAMS = {