I have a feeling everyone is rushing to answer this and once I post there will be 4 other replies...
;D
Its in the Description.ext in your missions folder. The .ext would have :
onloadmission = "Get Ready"
or is that just default if you don't change it?
Anyways, you can make it what you want.
Like:
onloadmission = "I love to take nice bubbly bathes"
and there's the
onloadintro = "this comes before intro"
Hope that answers your question.
EDIT: Damn ;D