You can select which country your exclusive port will use by adding additional info to your authentication.
Simply add the country code and a dash '-' to your username field.
For United States proxies you would use:
Your password remains the same
To do this in cURL you would use the following command:
curl -U US-USERNAME:API-KEY -x private.residential.rotating.proxyrack.net:10000 ip-api.com/json
This will bind and use a United States exit node when routing your request.