By default, the @RegisterRestClient configKey property is the sanitized name of the file containing the OpenAPI spec. For example, if the file name is petstore.json, the configKey will be petstore_json:

/* omitted */
@RegisterRestClient(configKey="petstore_json")
public interface DefaultApi { /* omitted */ }

If you want to use a different configKey than the default one, you can set the quarkus.openapi-generator.codegen.spec.petstore_json.[config-key] property.

Using the config-key the extension allow you to define all allowed properties with quarkus.openapi-generator.codegen.spec.[my_custom_config_key].* prefix. For example:

quarkus.openapi-generator.codegen.spec.petstore_json.config-key=petstore
quarkus.openapi-generator.codegen.spec.petstore.additional-api-type-annotations=@org.test.Foo

With it, you will have the following result:

/* omitted */
@RegisterRestClient(configKey="petstore")
@org.test.Foo
public interface DefaultApi { /* omitted */ }
If you configure the property config-key, it will override the sanitized file name (will not consider the order of the configurations). For example, having the following configuration: 
quarkus.openapi-generator.codegen.spec.petstore_json.config-key=custom_config_key
quarkus.openapi-generator.codegen.spec.custom_config_key.additional-api-type-annotations=@org.test.Foo
quarkus.openapi-generator.codegen.spec.petstore_json.additional-api-type-annotations=@org.test.Bar

The generated code will be:

/* omitted */
@RegisterRestClient(configKey="custom_config_key")
@org.test.Foo
public interface DefaultApi { /* omitted */ }