Custom roles do not appear in author dropdown

just to confirm, is this when creating events and under ticket form ?

It’s from the backend after an event has been created and you try and edit (quick edit or regular edit) to update the event author.

here’s another snippet which may help you with this – https://pastebin.com/dg299PuW

As for where to paste this : https://wp-events-plugin.com/tutorials/how-to-safely-add-php-code-to-wordpress/