'Drill Through Definitions' Dialog -> Target Report Tab
There is a dropdown at the bottom called 'Display Prompt Pages'. In this drop down select 'Only when required parameter values are missing'.
Your target report's 'Required' parameters should all be parameters being fed in from the Source report you are drilling through. Any parameters in the target report that are not being fed in from the drill through report need to be set as Optional.
If the prompt page is still appearing, you have one of your variables not being passed between drill through to target, or you have an extra required parameter in the target report that needs to be set to optional.
One way to diagnose this is to Hard Code all of the parameters in the target report except for one. Setup drill through for that one value and make sure it works without a prompt page when drilling. Once you have that one variable working, add in remaining varaibles from the source. If you are still having problems, you may need to add the variables in one at a time until you determine what is causing the problem.
Often just recreating the drill through you will notice a parameter you missed and catch it the second time.
IBM Guide: Create a drill-through definition