public interface CrudControllerBase<S extends Service<?, ?>,  SRC extends CustomQuery> {
    // some code
}

public interface ExistsControllerBase<S extends Service<?, ?>, EQUERY extends CustomQuery> {

@PostMapping("exist")
default Response<Boolean> exist(@requestbody PostBody<EQUERY> req) {
    // some code
}

}

@RestController
@RequestMapping("/v1/{tenant_id:[0-9A-Z]+}/user")
public class UserAdminController implements 
    CrudControllerBase<AdminUserService, AdminUserSearch>,
    ExistsControllerBase<AdminUserService, UserExistsQuery> {
    // some code
}

the "requestBody" in "/v1/{tenant_id}/user/exist" path get incorrect schema: 
    "schema": {
         "$ref": "#/components/schemas/PostBodyCustomQuery"
      }
the correct schema should be:
    "schema": {
         "$ref": "#/components/schemas/PostBodyUserExistsQuery"
      }