If you happen to have proxies from multiple countries but only want to use the ones from a specific country on your list, don't worry! It's actually really easy to do:
Simply select Backbone connection and use the country code in the username field (for example, "US" for USA proxies or "GB" for UK proxies), and you'll be good to go.
Example: Username-GB
Example if you're using Rotating Proxy Endpoint: Username-GB-rotate
To find the country codes, just filter the Proxy List by country, and they will appear in the username field: