Thanks for replying.
The report has two parameters (month and year). I'm trying to default month to last month and the year would be the associated year of the last month.
So I'll have a formula to calculate the last month
something like
month (dateadd("m", -1, currentdate))
So when the report is executed, it should default the "month" parameter to my calculated last month.
Something of equivalence to
{?month}} = month (dateadd("m", -1, currentdate)) //but this value must be set before user select parameter value and the user must see the default value as the calculated month.