I believe you cannot "add" a specific input disturbance (du) into the control system, just like you cannot "add" a specific reference signal (r). When you open the Control System Designer app and initialize the control system, the step response from the reference signal (r) to the measured system output (y) is shown on the bottom right tile (refer to Figure 1).
If you wish to view or evaluate the input disturbance rejection capability, you can select "IOTransfer_du2y," right-click it, and choose the option to plot the "step" response (refer to Figure 2). The step response due to the input disturbance (du) will be displayed, as depicted in Figure 3.
Figure 1
Figure 2
Figure 3