Please find attached a basic working PLECS Standalone example showing how you can calculate the peak-peak magnitude of a signal. You can copy the enclosed subsystem block into your own models. Note that you just need to set the averaging window time based on the frequency(s) of your signal of interest.
To open this model in PLECS Blockset, add an empty PLECS Circuit block in a Simulink model and from the File menu, choose "Import from Standalone".
Let me know if you have questions.