Run configuration will not work under 2026.2.2

I have an app which is an extension of a third party app.  I used to build using ANT and then debug it by specifying the third party app as the target and when it loaded my app I could debug it as normal.
I have upgraded the build to Gradle and tried to run it, however, this does not work.  In the run configuration I have the third party app defined as the target but now Idea can not find it.  Even though I include the appropriate jars in the classpath (using ‘Modify classpath’) the target class can not be found.
Is there something in Gradle that Idea uses to determine its class path?  

I also have the same app under an ANT build where Idea can at least find the third party app.  This leads me to the conclusion that it is Gradle that is affecting the run configuration.