Return transactions based on External ID

Hello

I am trying to do a query to return transactions based on External ID with postman using GET Method

GET (URL)/api/rest/2.0/data/customObject/xx/instances?='UniqueCode=External ID 123' 

In this case, my unique code is a specific external ID (External ID 123), but the responses give me the WHOLE DATA in the CDO including External ID 123.

So I get External ID 456

             External ID 789

             etc...

Any ideas of how can I just capture data information for External ID 123 only?

Thank you