There is no way to manually submit to Castro. This is because Castro reads off of the Apple Podcasts directories, so as long as your podcast is in Apple Podcasts, it will appear on Castro too!
How to Find Your Castro Link
Find your Apple Podcasts ID. It is the number at the end of your Apple Podcasts link.
Example: https://podcasts.apple.com/gb/podcast/create-reach-inspire/id1449965359
So your ID would be 1449965359.Type in https://castro.fm/itunes/ITUNES_ID
Example: https://castro.fm/itunes/1449965359
This is your Castro link 👍