Struggling to populate a group's Data when initialy empty

Hi, I’m stuck here :
I am making a simple accounting app.
In my Data Types, I have Records, and each Record has several Lines
This Popup Type is Record. The Table Type is Line (The record’s lines …)
Here is an accounting record I want to modify, by creating a new line.

However, the new line group (which is of type Line) data is empty at this stage, and I cannot figure out how to populate it with the fields I filled, or to create it …
If I try ‘Make change to thing’ to this group’s data, it will not work (because empty ?) …

I have attempted to use ‘Custom States’ but again, I did not figure out how to populate a Custom State when it has a Data Type (i.e. made of several fields).

Any clue ?