You only enter the airplane's measurements. What measurements you ask? The ones shown on their diagram.

You measure the wing:
halfspan
tip chord
root chord
LE sweep

You measure the horizontal tail:
halfspan
tip chord
root chord
LE sweep

You measure from the LE of the wing at the root to the LE of the tail at the root.

Plug those numbers into the application's page and click the "compute" button (or whatever the button is called).