Unable to get the access token for Adobe api

Hello everyone,

I have been able to get the access code using this URL:

https://secure.na3.adobesign.com/public/oauth/v2?redirect_uri=https://localhost/AcrobatSignOauthServer/Home/OAuthCode&response_type=code&client_id=redacted&state=xxxxxxxxxx&scope=agreement_read:account+agreement_write:account+agreement_send:account+widget_read:account+widget_write:account+workflow_read:account+workflow_write:account

However, when I use the access code returned from the URL above to get the access token, I got the error "Error: Request Security Violation at /oauth/v2/token". I tried with different versions of the URL below but still unable to get the access token:

https://api.na3.adobesign.com/public/oauth/v2/token

https://secure.na3.adobesign.com/public/oauth/v2/token

https://api.na3.adobesign.com/oauth/v2/token

https://secure.na3.adobesign.com/oauth/v2/token

Can someone tell me what should be the correct URL for the endpoint? I checked on the developer forum  and quite confused by different answers and none of them worked for me. Thanks.