CORS error when getting access_token

From my reactjs app, I get request_token in my Redirect URL after executing the login API. After getting request_token, I run another API to get access_token. This gives me a CORS error.
1. I tried adding the header in POST, in package.json and also in my NGINX, but I still get the CORS error.
2. Should my URL be whitelisted from your side?
3. Appropriate CORS access headers needs to be added from your server so that I can get it in my browser.
Appreciate your help. Thanks.
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