What expression should I use with a Domain query to pass a BigDecimal parameter to a subreport?

[Mark.Casey]

I am using Jaspersoft Studio 6.1. I have a main report (project level) and a subreport (task level).  I need to pass a BigDecimal INVESTMENT_KEY parameter to the subreport so that the subreport will look only at tasks for the current project in the main report.

The data is pulling via a Domain Query, so using SQL with a where clause is not an option.    I have created a Parameter1 with class java.math.BigDecimal to handle the INVESTMENT_KEY, but what expression should I use to capture the BigDecimal parameter?