Hi,
If you are using a dataservice in your transformation along with a Table input step you can define push down parameters. Those parameters can be further push downed into queries, in the way that you are requesting help. Look at this link for more information: Develop and Optimize a Pentaho Data Service - Pentaho Documentation
Since you are using 7.1 version you are probably using a standard SQL query over a dataservices datasource from CDA, on which you can then define the push down parameter value by means of stating it's value in the WHERE clause of the CDA datasource in your dashboard.
Let know if you need further help...
Best regards