If you have signal processing toolbox, you can use square() function to generate the waveform.
t = linspace(0, 5, 500);
y = square(t*2*pi);
If you don't have the toolbox then use
t = linspace(0, 5, 500);
y = mod(t, 1) > 0.5;
and then plot it on UIAxes object
plot(app.UIAxes, t, y)