It isn't supposed to change. The text you write down is the text you write down.
Instead of using hardcoded values, you can tell loadrunner to use the contents of parameters by putting the name of a parameter in there, delimeted by accolades -- { and }
So:
"Value=020;338;11/20/2013"
becomes:
"Value={flightnumber};{price};{flightDate}"
Of course those parameters do need to exist up front.