FPScript offers you the option of programming your own functions. You can create these as you would regular formulas; however, you need to use the Arguments statement in the first code line to declare arguments for the function.
1) Create a new FPScript formula:
Arguments data, pretrigger
Dim idx = SearchValue(data.x, pretrigger)
Signal(data.y[idx,-1], data.x[0, -idx])
The result of the SearchValue function is the position (index) of the pretrigger value.
The index operator  exctracts a section from the signal.
2) You can now use this function in other formulas. You only have to click on the data sets to which you would like to apply the function, use your mouse to select them from the Object List and drag them onto the function. You can then use this new formula for further calculations or for displaying in documents.
See also the FlexPro Help
Analyzing Data Mathematically
Applying Formulas to Data Sets