When filtering on a Name Query API using the example below am not getting relevant records instead it's displaying all the records from the API. Can some take a look and let me know if am using the right syntax? api/rest/named-query/DWorxReferenceData/FieldCountry/GD?/?$f=FieldToCountries.Country.CountryID%20=%20%27COU0000000016%27 

Am trying to search on the Attributes which is added to the Named Query using the Multi Record Expression.

You won't be able to write the filter like this.
The filter is a condition which needs to be a valid SemQL expression.

I don't think it's possible to construct SemQL for a multi-record expression.

Take a look at our documentation on Named Query Parameters:
https://www.semarchy.com/doc/semarchy-xdm/semsq.html#named-queries-parameters