Crystal Reports Passing values

Message

     Hi,

In my main Report have two created formula fields as in the following:

Shared NumberVar CurrentID:={usp_CandidateMailMerge;1.CandidateID}

and the second is :

Shared StringVar Letter:=Filename

I have created in my Subreport the same two variables as in 

Shared StringVar Letter;
Letter;

Is there a way of setting a parameter value in my Subreport equal to the Shared Variable i.e
{?FileName}=Letter

Thanks in advance