CastBox will eventually pick up your podcast feed itself. There is no way to manually submit to CastBox.
Add a CastBox subscribe button to your podcast page
When you get confirmation that your podcast has been accepted you can add your CastBox URL to display an CastBox subscribe button on your Audioboom podcast page.
If you see any unusual errors you're unable to fix yourself, please create a support ticket and we will do our best to help.