If you're using Hermes engine, you can simply setup a 'Run configuration' in Webstorm. To do this, go to Run -> Edit configurations and setup a React Native run configuration and name it whatever you like i.e Debug.
Make sure to select 'Hermes engine is enabled' and unselect 'Build and launch application' and click 'Ok'.
After the run configuration is setup, run your app as you normally do. Set any break points on the lines you want to debug and then on the top right corner of the IDE, select your newly created Debug run configuration and click on the Debug button next to the Run button. This should now start the debugger and trigger the breakpoints within WebStorm.
https://youtrack.jetbrains.com/issue/WEB-53699/React-Native-Debugging-JS-on-Hermes-using-WebStorm