Get parameter dynamically SSRS

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SSRS

SQL-Server-2008R2

SSRS-2008

Hi,

I am creating a subreport and trying to load the child report dynamically.
e.g :
it should not be loaded on the 1st load.
it will get loaded once i click on the required field in the master report.

For example:
if i will click on country name column in Row 2 then child report should get filtered with the input.

Now i am using "=First(Fields!CountryName.Value, "DataSet1")" in my parameter expression.(which is not correct)

Thanks,
Biswabid