Crystal Reports 2013 Standart - problem to link date formula to subreport.

Hi. I am using a subreport to populate a stored procedure. In main report I am using funcitons and custom parameter and link them to subreport. I can link date parameters to subreport but can't link formulas. This is the function:

if(HasValue({?BaslangicTarihi}))

    then {?BaslangicTarihi}

else

    CurrentDate

The type of subreport parameter is Date, so the {?BaslangicTarihi}.

{?BaslangicTarihi} is a custom parameter in main report. I can link it to subreport parameter @fD but can't link the formula.