could you provide more details?
Note To Supplier in which form?
I assume that, you have entered note to supplier at PO level.
Then when you query with that PO number in Inventory Receiving Transactions form. There in comments field you need to default the value.
For that do like below.
Create one personalization with WHEN-NEW-ITEM-INSTANCE Trigger and RCV_TRANSACTION.LINE_CHKBOX as trigger object.
in actions type as Property
Object type: Item
Target Object: RCV_TRANSACTION.COMMENTS
Property Name: Value
Value: =select note_to_vendor from po_headers_all where po_header_id=:rcv_transaction.po_header_id
Hope this will help you...