How to create child items for a subgrid element in Power Pages

Today we bring something a little more complex.
How can I create sub-element that belong to the selected one in Power pages? Like all the deliverables from a specific project?
How do you create a pop-up window in Power Pages that allows you to create or modify elements from a child table of the current table.
We have multiple manners, but as this one will be rather long, let me show directly.
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