AWS Cognito token endpoint returns 405 Method not Allowed (Angular)

When a user logs in via an SSO they are redirected by to my auth page where I'm grabbing the authorization_code and attempting to authorize the user to grant access.

let headers = new HttpHeaders();
let auth = btoa(<clientId> + ':<client secret');

headers.set('Content-Type', 'application/x-www-form-urlencoded');
headers.set('Authorization', `Basic ${auth}`);
headers.set('Accept', 'application/json');

this.http.post(
'https://<cognito domain>/oauth2/token',
null,
{
headers,
params: new HttpParams()
.set('grant_type', 'authorization_code')
.set('code', <code>)
.set('redirect_uri', <redirect url>)
.set('client_id', <clientId>)
}
)


I've also done this as a body rather than params and I get the same error. Is there something I'm missing?

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