digital.nhs.uk/developer/api-catalogue/patient-flags-service---fhir-api#get-/PatientFlag
There is no such resource as PatientFlag and no such endpoint is allowed.
Also the mime type specified is wrong for FHIR: application/json. It needs to be application/fhir+json
FHIR can be extended in conformant ways, but not in the way proposed.
But is custom behaviour needed?
This, as proposed: GET /PatientFlag?patient=9449306753
is possible in a FHIR conformant way already by just doing this:
GET /Flag?patient.identifier=9449306753