Is there a way I can check in dialogue manager
that if a parameter '&PARAM01' is passed
from URL then assign the relevant value.
But when it is absent then set the value to something else.

Since this is passed from URL I do not want it
to set using
-DEFUALT &PARAM01 = 0;
( Otherwise it will popup in front end screen )

Thanks

Try something like

-IF (&VAR.EXISTS) GOTO SKIP;
-SET &VAR='VALUE';
-SKIP

Thanks JG