You can use the Simulate Signal Express VI, which is part of the Waveform Generation Sub-Palette. The signal Type should be Square.
The block diagram will be as shown below
Note: This image is a LabVIEW snippet, which includes LabVIEW code that you can reuse in your project. To use a snippet, right-click the image, save it to your computer, and drag the file onto your LabVIEW diagram.
The Front Panel will be as shown below