My scripts transform whispers into what are known as ‘queries’ in the context of IRC. As such, you can use mIRC’s native handling of queries to handle whispers:
/help /msg
/help on Open
/help on Text
If you don’t want the window to appear, you can use:
;; triggers when a whisper is received
on ^*:OPEN:?:!trgger:{
;; adds the message to a custom win buffer; done for example purposes
/aline @CustomWIndow $nick $+ : $1-
;; Halts mIRCs default query window from being created
halt
}