In order to open the command in the PAK editor, a JAR file must be created. This can be done in three different ways:
You can create the JAR by executing the gradle MetaJar command in the terminal.
Or you can navigate to the build.gradle file and execute the MetaJar task there by clicking on the green play button:
Or use the Gradle toolbar on the right side of the page
After executing one of the above options for creating a JAR file, you can find a libs folder with a JAR file in the build folder that was created as a result: