Copy subitem data down, in a column, when creating new Subitem

In my Book Inventory Board, I have a subitem column to enter the name of a Book. I have to enter this name everytime I enter a new subitem, so that it counts it in the inventory. Any way to automate that?

EXAMPLE: Main Item name is: PAV-1

All subitems created under this main item, the Book Name column has to include PAV-1, otherwise it won’t count it for the inventory. Currently have to click on the cell and type it.

Thank you!!