How can I edit the WordPress menu link to allow fb: instead of http

It’s not your theme that’s preventing menu items from being saved with fb: protocol. It’s WP Core. WP Menu items are intended to be web URLs, because visitors can typically access a WP site from any device – not just a mobile device. Even a mobile device that doesn’t have the Facebook app installed would not recognize a fb: link.

My suggestion would be to create a menu through your theme through some other means – not a WP Menu, but programmatically or even hard-coded, depending on your site’s needs.