如果表单服务在Symfony3中扩展另一个服务，如何使用表单作为服务？

How can I use form as a service if my form service is extending another service?

For example if I have parent form service like this:

project.modelbundle.parent_form_type:
    class: Path/To/Parent/Form/Class/name
    calls:
        - [callMethod1, ["arg1"]]
        - [callMethod2]

And form that extends service above that I want to use in controller:

project.modelbundle.my_specific_form_service:
    parent: project.modelbundle.parent_form_type        
    arguments: ['@service_container', '@request_stack', 'some_string_arg']

How can I use defined service project.modelbundle.my_specific_form_service as form in controller?