Currently, therapists struggle with interaction of rehabilitation robots due to non-intuitive interfaces. Therefore their acceptance of these robots are limited. This paper presents the development of ARMStick, a lightweight and small robotic interface in the shape of a human arm with 4 actuated and 3 unactuated joints, to facilitate the interaction between therapists and rehabilitation robots. It allows therapists to intuitively perceive joint-dependent data as recorded by rehabilitation robots, and teach poses and trajectories to individualize therapy to the patient. It's range of motion (RoM) covers the RoM of a healthy human. The device's measuring accuracy of CI 95% $ \lt \pm 0.322^{\circ}$ and movement accuracy of CI 70% $ \lt \pm 5.23^{\circ}$ lie within the confidence interval of average visual perception. A demonstration of the device to 5 therapists indicated that it could indeed improve efficiency and efficacy bottlenecks in current robot-assisted therapy. Comparison of ARMStick to two visual user interfaces showed a decrease in mean adaptation time from 15s to 5s for three arm configurations presented to the therapists.