You'll find a lot of advice I gave in this Academia Stack Exchange answer useful, though it relates to R scripts and LaTeX. Unless the script is available from a very stable repository, only citing the praat script may not be helpful in a few years time. It's better to ask the original author if you can republish it as an appendix, or make all your analysis scripts available in a stable repository.
Caveats out of the way, the APA citation style indicates how to cite software. The APA style blog gives advice on how to do so, and following that I would recommend a citation like:
Smith, J. (2014). Name of Script or Function [Praat script]. Retrieved from https://www.example.com/praatScripts