not able to use the expression in forms personalization

 Hi I am trying to use the built in execute procedure  option to call a custom procedure.

and when passing the expression ${item.po_defaults.ship_to_organization.value} as a prameter its giving some error like, could not be evaluated because of error ORA-00923: FROM keyword not found where expected.

Can any one share your experience in R12, if there is any restriction to use these expressions.

I have tried 

begin

XXUS_TEST('ac', ${item.po_defaults.ship_to_organization.value});

end

also 

='begin

XXUS_TEST('ac', ${item.po_defaults.ship_to_organization.value});

end'

where as 

begin

XXUS_TEST('ac','AC');