Using custom actions in YAML configuration

Hi all,
I am trying to use our custom action in YAML project description.
Something like this:

user_paths:

• name: GenerateTrackingMessages - Belonging to accounts
actions:
steps:
• custom:
actionType: MyActionName
name: my_action_name

But I can't find any documentation about the topic.
Does anyone know how to use custom action in YAML project description?