Crystal reports - How to set data source for subreports?

I could set the datasource for the subreport which is using ttx files, but my requirement is that i need know the mapping field value.

Out of the many records i display, i need to know the record on which user clicked( i.e drill on subreport) to view its details( the details are shown in subreport).

Actually this could be achived by linking the field(usually unique field) in main report to a field in subreport. But as i mentioned above, my requirement is to know the value of that field.

Thanks.