How to resolve a Cors error when trying to get an authentication token using authorisation code grant in Next.js

I have an app where im trying to send a generated pdf to get a signature. Pretty straightforward.
I got the authentication working in postman, but when I try it in code, I get this error:


Access to fetch at '
https://account-d.docusign.com/oauth/token' from origin '
http://localhost:3000' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.


I can retrieve the oauth code with this:
if (router.asPath !== router.route) {
setOAuthCode(new URLSearchParams(window.location.search).get("code"));
}



but in a seperate use effect, i try to submit this oAuth code, it falls appart:
useEffect(() => {
const getAuthToken = async () => {

const tokenUrl = "
https://account-d.docusign.com/oauth/token"; /> const encodedSecret = btoa(
`${process.env.NEXT_PUBLIC_DOCUSIGN_INTEGRATION_KEY}:${process.env.NEXT_PUBLIC_DOCUSIGN_CLIENT_SECRET}`
);

const headers = {
"Authorization": `Basic ${encodedSecret}`,
"Content-Type": "application/x-www-form-urlencoded",
};
const body = new URLSearchParams({
grant_type: "authorization_code",
code: oAuthCode,
});

const response = await fetch(tokenUrl, {
method: "POST",
headers: headers,
body: body,
});

if (!response.ok) {
throw new Error(
`Failed to get access token: ${response.status} ${response.statusText}`
);
}
const data = await response.json();
setAuthToken(data.access_token);
};
getAuthToken();
}, [oAuthCode]);



Any kung fu would be greatfully accepted