You can use $1 to refer to the trigger hello:

on 1:TEXT:Hello:#: /msg $chan $1 to you, too!

As basic and classic as it seems, it's also susceptible to trigger flood attack, and its limitation is confined to one word "Hello." Not to mention it may or may not be triggered if someone else uses a custom script with control codes added, while your mIRC is not set to strip out incoming messages.