Was it just about the content-type?!

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') {