The only way to do that is to install the PhoneGap CLI. You can then navigate to your project directory and type "phonegap serve", and you should be able to use the app in the PhoneGap Developer App.
Do keep in mind that the PGDev app only includes Core + a few 3rd-party plugins and does not always accurately reflect what your app will do when actually built.
That said, PGB gives you an installable app -- what's the use case here for using the PhoneGap Developer App in your situation?
So I still need an apple developer certificate to do this?
If you're using the PhoneGap Developer app, no. Just keep in mind that it is not a true test of how your app will function when it is running standalone. The Developer app, by virtue of itself being an app (and a PhoneGap app as well), has many edge cases that your standalone app may not encounter. As such, you will need to get a dev cert from Apple at some point so that you can test the app on your device without the Developer app. (And, TBH, the sooner the better.) You'll need it anyway to submit to the App Store.