Pass numbers with Datagrid custom cell

Good day,

I am trying to pass numbers that get entered into a Datagrid via a custom cell and an input box.
I have recorded a Loom video that explains exactly what the issue is that I am facing.

The short version is: I need to be able to add custom data to the Selected Rows of a datagrid by using a custom cell in the datagrid and an input box.

Loom video with full explanantion: Loom | Free Screen & Video Recording Software | Loom

I really appreciate any assistance!