Go to user settings and enable the 'Enable Custom Project Template'.

Create a project template.

Assign the user and set a description for each task. Set task as per template requirement. Only 'Active' templates are able to be set.

When clicking on 'add a line' it opens the wizard of tasks, fills details, and saves it.

To add the sub-task click on the sub-task option.

After that, the 'Sub Task Template' wizard will appear. Add the details.

By adding a template to the project template, tasks, and sub-tasks are automatically added to the project.

Here, you can see the smart button of the 'Tasks'.
Click on the task's smart button.

Kanban view of the task. Here, you can see the task with sub-tasks and stages.

In the project settings, Select the 'Default Project Stage' of the 'New Project Configuration'.

Create a new project and select the project template. To create a task, click on the 'Generate Task' button.

You can see the task is automatically created with default stages as per configuration.

The sub-tasks are visible in the task's form view.