Hi J u n i o r,
Just to be clear here, in order to support this, the REST API method you are consuming needs to support it - it's the provider that defines the available functionality, not the consumer.
If the provider does support receiving a list of country codes for that method, then most likely it will be somewhat similar to
@Marco Mateo's suggestion above, but you need to check the REST method documentation from your REST API provider.
Hope this helps!