How can I calculate stall speed using foilsim?
All you can do with any airfoil analysis software is to calculate the stall angle of attack and lift coefficient. To get to the stall speed, you need to have knowledge about the wing loading and wing shape of the particular aircraft you need to calculate the stall speed of. For a single airfoil no stall speed can be defined - this is only possible for a specific airfoil on a specific airplane.

Foilsim allows you to add this information via the "size" button. Next, click on the "shape" button and increase the angle to the maximum where the flow on the upper side does not yet separate from the airfoil contour. If you now change to the "Flight" button, you can enter the speed and altitude. Select "Lift" on the right side and see how the lift changes with speed.

If you divide the lift by the gravitational acceleration of the selected planet (yes, you can run Foilsim on Venus or Mars, too - on earth that would be 9.806 m/s²), you get the mass of the aircraft that will stall at the selected speed.

However, since this is a very simple simulation that is not meant for actual aircraft design, and the airfoil shape cannot really be selected beyond camber and angle of attack, I advise you to not build an aircraft with those results. The fact that the stall angle of attack is always 10°, regardless of the airfoil contour used, should be warning enough.