In what exact context do you need this variable to receive this value?
Does this "calculation" take place when a script executes due to pressing a button? (the name of your variable suggests that this is the case.)
If so, Set Variable can be set up to assign True to the variable:
Set Variable [ $$pbutton1 ; Value: True ]
If you do need to do this inside a calcualtion, the let function can assign a value to a variable:
Let ( $$pbutton1 = True ; Put rest of your calculation here )