Here is an example of how it is possible to capture the FFT at a certain moment. We will make a setup file where the formula will save the FFT at the click of a button.

First we need a user input channel, which will be controlled by a button.

After the channel is created we create a formula which has FFT mathematics for the input channel. The time axis on the formula needs to be changed to Async and the channel from user inputs ("store FFT") needs to be chosen.

The async formula will show the latest FFT block as soon as you will push on the button and will not refresh automatically.