Validate XML message against XSD schema

I'm trying to validate a %Stream.GlobalCharacter with an XML content against an XSD schema, to validate mandatory nodes, etc...

I'm trying to use the Ens.Util.XML.Validator class, as suggested on this post:

https://community.intersystems.com/post/validate-xml-message-against-xsd...

However, when calling the ValidateStream method, if schemaSpec is left empty, it just validates that is a "valid XML".

When specifying anything on schemaSpec, i receive the error "SAX XML Parser Error: Unable to Resolve SystemId"

Any examples  with the right specification on schemaSpec for ValidateStream? (a filepath to the XSD file, for example, or an XML schema imported on the namespace)