Custom spring initializr instance - adding custom java class

Question

I am trying to create my own instance of spring initializr as described in https://docs.spring.io/initializr/docs/current/reference/html/#create-instance. My need is to add custom java classes in the generated project (Other than the default @SpringBootApplication class). I am not able to figure out how to do that. Not finding any examples anywhere as well. If someone can guide me on how to do that would be helpful.