An embodiment of a hearing assistance apparatus for performing a Real Ear Measurement (REM), comprises a hearing assistance device housing, a microphone within the housing, an earhook connected to the housing, and a flexible tube. The house has a first opening for guiding sound into the housing to the microphone. The housing and the connected earhook form an interface, where the earhook has a shape to provide a slot near the interface of the housing and the earhook. The tube guides sound, and has a first end and a second end. The first end of the flexible tube and the slot of the earhook cooperate to retain the first end of the flexible tube in the slot of the earhook and flush with the housing to provide a sound-tight connection with the first opening.