Business need 

I want to be able to create a private sub-task of a main customer project that is initially shared to him via the Customer portal. 

Constraint 

Via the Customer pack parent task I have enables the QS Project sub-tasks, that are all related to child project "[QSBE] "name of project" ". This child project and all sub-tasks linked to it are visible by the customer using his Customer Portal. 

Additionally to that, I want to create an additional task, internal to my company (like Odoo), so I can input some information that is not shared with the customer(some notes, links to Google Drive not to be shared with the customer, etc). 

Solution

1. Go to your main task (like the Customer pack project) 
2. Click on "Go to QS Project" 
3. After keeping your tasks that need to be shared with the customer (remember, [QSBE] "name pack" project is visible on customer portal), you create a new task
4. Leave project field empty, select your customer, go to "Extra info" tab and select your company on "Company" field. Make sure the "Parent task" tab is selected. 
5. Now you have on your list view a new task saved as "Internal"