Current samples for federated queries in SPARQL with blazegraph suggest the following format:
SERVICE <http://data.cervantesvirtual.com/openrdf-sesame/repositories/data> { .. }
or
SERVICE SILENT <http://data.cervantesvirtual.com/openrdf-sesame/repositories/data> { .. }
Following the discussion here, it seems that the following should be valid as well:
BIND(uri("http://data.cervantesvirtual.com/openrdf-sesame/repositories/data") as ?endpoint) SERVICE ?endpoint { .. }
or
?item wdt:P123456 ?endpoint . SERVICE ?endpoint { .. }
Obviously, provided that the endpoint is accessible from query.wikidata.org. If not:
?item wdt:P123456 ?endpoint . SERVICE SILENT ?endpoint { .. }