First, it is essential that you make sure you have proper copyright permission for any music you use in your podcast. Podbean offers a few preloaded music options in the app, which are covered for podcast usage.
For music you have secured copyright permission for, you can sync the music from iTunes on your computer to your iphone by referring to Apple’s help link here: https://support.apple.com/en-us/HT201253. Once you’ve done that, simply select it as your background music when recording in the app* (press + on the recording page to select the music).
*Currently, this feature is only available in the Podbean iOS app, not the Podbean Android app.