how to assign year and month in YYMM format as a default value for input parameter in Calculation view?

my period parameter is not mandatory, if the user is not giving any input then the current date in YYMM format should be defaulted e.g: 1604. Something like

if p_period is null then

   select to_varchar(now(),'YYMM')

   INTO v_period

   from dummy;

  else

    v_period := p_period;

  end if;

But the above is not working in a scripted calc. view, when i am using the same in procedure in catalog folder its working as desired. I want to achieve the same functionality in calc view.