Humm, I never use lsftp, but you should use a label for the drop triger.

It's quite simple
FTPSendLabelOnDrop !LSFtpSend

You have to use mzscript and put into your script file
*Script Bang !LSFtpSend
*Script Exec !FTPSend "%{args}" "%{RemoteFile}"
*Script ~Bang

I guess you had read the lsftp docs and you know how to connect, etc...