To insert a detail line in between the items of a MoneyWorks unposted transaction, such as Sales Invoice, Purchase Invoice, Payment or Receipt transaction; first you add a detail line (Which will be added at the last item), then position the cursor over the newly added detail line, hold down the SHIFT + CTRL keys (for Window) or OPTION key (for Mac), and drag the detail line to a new position.
In the above example, I moved the item BC200 to a new position, which are in between the item BB100 and CA100.