Again, the best option will be talking to your board manufacturer,
To build a castellation the edge needs to be exposed, before the plating pass, meaning atleast some level of routing prior to plating (Its roughly like how a plated slot is made)
This out of order routing is what will generate extra costs,
The other thing is making a clear description of what you want on a mechanical layer, and again in the notes to the fabricator. If your doing something weird, expect them to need guidance