Was it just about the content-type?!

src/components/molecules/EmailForm.svelte

@@ -33,7 +33,7 @@
     const toggleFocus = () => inputInFocus = !inputInFocus
 
     // Handle form submission
-    async function handleForm (event: SubmitEvent) {
+    async function handleForm (event: Event | HTMLFormElement) {
         const data = new FormData(this)
         const email = data.get('email')
         console.log('form:', data, email)
@@ -41,9 +41,10 @@
         if (email) {
             const req = await fetch(this.action, {
                 method: 'POST',
+                headers: { 'content-type': 'application/json' },
                 body: JSON.stringify({ email })
             })
-            const result: FormStatus = await req.text()
+            const result: FormStatus = await req.json()
             formStatus = result
 
             // If successful

src/routes/api/newsletter/+server.ts

@@ -19,15 +19,14 @@ export const POST = (async ({ request, fetch }) => {
     // Newsletter API request
     const req = await fetch(`https://emailoctopus.com/api/1.6/lists/${NEWSLETTER_LIST_ID}/contacts`, {
         method: 'POST',
-        headers: { 'Content-Type': 'application/json' },
+        headers: { 'content-type': 'application/json' },
         body: JSON.stringify({
             api_key: NEWSLETTER_API_TOKEN,
             email_address: email,
         })
     })
-    const res = await req.text()
+    const res = await req.json()
     console.log('server:', res)
-    return new Response(res)
 
     // Other error
     if (res && res.status !== 'PENDING') {