Hi @Ansi,
You’re not using the correct Asana element to achieve what you want. What you’re referring to is a custom task type. I.e. it’s not the resource_subtype you set on a task, it’s the custom_type property. Setting the task to a custom type will get you the UI equivalent you’re looking for.
However, there is a catch; currently you can’t set a new task to a custom type when you first create it. You have to first create it as a normal task, then immediately update it to be the desired custom type.
EDIT:
To clarify, you do set the resource_subtype to custom as part of the process of updating a task to be a custom type. But you also have to set its custom_type property. And again, just for now it won’t work on a new task; only to update an existing task.