FFT <Size> points
Description Init radix <Size>
  See fftinit command. Radix is log base 2 of Size.
Hot Key  none
Script fftinit(radix)

Open scope and paste this script into the command window.
Then press <Ctrl + R> to plot a square wave

Select the Calculator menu "Build/Pulse/Complex/FFT init/FFT 256 point. then press f to get the 256 point transform.

numpoints = 256
time = (vector(numpoints)/(numpoints-1)*1m)
newplot pulseplot time
setplot pulseplot
y = 2*pulse(500u) - 1
plot y