ProductsTable.selectedRow, which has all the column values of the selected row. By referencing
productNameon it, you're accessing the value of productName column. By setting Default Text to this, you're pre-filling the form with this value.
selectedRowproperty. To see the exposed properties you can access for any given widget, check the Properties section of its entry in the Widget documentation.
ProductsTable.selectedRowcan be accessed only in other widgets, queries, and APIs of ProductListPage.
ProductsTable.selectedRowcan't be accessed from any widget, query, or API of AddProductPage.