There isn’t any ‘correct’ wording.
You can put something like:
Congratulations
Congratulations on your Baptism
Or just have a cross on the cake or a shell (symbol of baptism) or a dove (symbol of Holy Spirit) and their name. Or get a cross shaped cake.
Really, whatever you choose will be fine.
Go to Google images and type in “baptism cake”. There are tons of pictures. Many are for infants, but some are more generic and would be appropriate for an older child or adult.