diff --git a/fern/custom.js b/fern/custom.js
index 6434797a4..5d9270499 100644
--- a/fern/custom.js
+++ b/fern/custom.js
@@ -158,12 +158,15 @@ function initializeSubscribeForm() {
     submitBtn.textContent = 'Submitting...';
 
     var formAction = form.getAttribute('action');
-    var formData = new FormData();
-    formData.append('email', email);
+    // Customer.io's submit_action endpoint expects application/x-www-form-urlencoded
+    // (the encoding native HTML form POSTs use). FormData sends multipart/form-data,
+    // which the endpoint accepts but does not record as a form_submit event.
+    var body = new URLSearchParams();
+    body.append('email', email);
 
     fetch(formAction, {
       method: 'POST',
-      body: formData,
+      body: body,
       redirect: 'manual',
     })
       .then(function (response) {
diff --git a/fern/custom.spec.js b/fern/custom.spec.js
index 8b4860837..0a725978c 100644
--- a/fern/custom.spec.js
+++ b/fern/custom.spec.js
@@ -103,6 +103,10 @@ assert(customJsContent.indexOf("redirect: 'manual'") !== -1,
   'custom.js uses fetch with redirect:manual to handle 302');
 assert(customJsContent.indexOf('opaqueredirect') !== -1,
   'custom.js checks for opaqueredirect response type');
+assert(customJsContent.indexOf('URLSearchParams') !== -1,
+  'custom.js sends body as application/x-www-form-urlencoded (URLSearchParams)');
+assert(customJsContent.indexOf('new FormData()') === -1,
+  'custom.js does not use FormData (multipart) for the Customer.io submit');
 assert(customJsContent.indexOf('subscribe-form-message') !== -1,
   'custom.js updates the message div');
 assert(customJsContent.indexOf('Thanks for subscribing') !== -1,