Getting status 403 when trying to get oAuth access token

Description
I’m trying to get an oAuth access token from a user, but I keep getting the 403 forbidden error when sending the POST request.

I have followed the tutorial at https://marketplace.zoom.us/docs/guides/auth/oauth religiously. I have even checked previous threads that talked about this error, but still couldn’t fix it

How To Reproduce (If applicable)
var fetchURL = ‘https://zoom.us/oauth/token?grant_type=authorization_code&code=dHrAQyHrTw_C1zkc5XgTO-fCRTjmsfWeg&redirect_uri=http://localhost:3000/

const response = await fetch(fetchURL, {method:“POST”, headers{ “Authorization” : "Basic " + encoded_cred}})

–> encoded cred is encoded in base64 and contains Client_ID:Client_Secret

Please help, thank you.