Consume a nopCommerce API behind OAuth2.0 - node.js

Question

My goal is to consume an API which has already been deployed with nopCommerce (I do not have dev access to the server - I am just a user). There is a sample client application here, but the code is in C#. I have a webapp deployed on an Azure server using node.js. The API uses the OAuth 2.0 Authorization Code grant type.
I did a bit of Googling and it appears that client_credentials is typically used for this type of server to server flow:
How does 2-legged oauth work in OAuth 2.0?
Using OAuth for server-to-server authentication?
There is also an answer here which suggests that I can manually retrieve a token and then store it on the server. This is what I'm currently doing while testing my code.
I also found this answer which appears to be asking the same question. Like the author of that post, I am able to get a token via Postman, but my node.js code fails.
OAuth2.0 for nopCommerce
I wrote a minimal example in node.js here.
import { config } from 'dotenv';
import * as path from 'path';
import fetch from 'cross-fetch';
const ENV_FILE = path.join(__dirname, '.env');
const loadFromEnv = config({ path: ENV_FILE });
export async function getCodeUrl() {
const params = {
client_id: <CLIENT_ID>,
redirect_uri: 'http://example.com',
response_type: 'code',
};
console.log(params);
const url = new URL(`http://example.com/OAuth/Authorize`);
Object.keys(params).forEach(( key ) => url.searchParams.append(key, params[key]));
const res = await fetch(url.href, { method: 'GET' });
return res;
}
export async function getToken(code: string) {
const url = new URL(`http://example.com/api/token`);
const options = {
form: {
client_id: <CLIENT_ID>,
client_secret: <CLIENT_SECRET>,
code,
grant_type: 'authorization_code',
redirect_ui: 'http://example.com',
},
headers: { 'content-type': 'application/x-www-form-urlencoded' },
method: 'POST',
};
console.log(options);
const res = await fetch(url.href, options);
console.log('res', res);
return res;
}
const test = async () => {
const codeUrlString = (await getCodeUrl()).url;
const code = (new URL(codeUrlString).searchParams.get('code'));
if (code) {
console.log('code', code);
const tokenResponse = await getToken(code);
console.log('token res', tokenResponse);
}
};
test();
I am successfully able to retrieve the authorization code, but when I use that in a POST request to get a token, I get this error:
{ error: 'invalid_client' }