You can add a note to a purchase order in the App, either to a new Purchase order or an existing one.
To create a new Purchase Order, go to Expenses > select the + symbol in the bottom right.
You can then add the note in the Notes section while completing the Purchase Order.
To add a note to an existing one, go to Expenses > select the Expense / Purchase that you want to edit > select Add Note > write the new Note and select Add Note. The new note will now be attached to the Purchase Order.