If you want to link the Item Receipt to an existing Purchase Order, there are two ways you can accomplish this:
// IMPORTANT NOTE: Be careful! This will link *the entire* Purchase Order because you used LinkToTxnID and not ItemLineAdd/LinkToTxn/*, see the note above!
<?xml version="1.0" encoding="utf-16"?>
<?qbxml version="8.0"?>
<QBXML>
<QBXMLMsgsRq onError="stopOnError">
<ItemReceiptAddRq>
<ItemReceiptAdd>
<VendorRef>
<ListID>70000002-1247235115</ListID>
</VendorRef>
<TxnDate>2009-10-03</TxnDate>
<LinkToTxnID>56-12573705936</LinkToTxnID>
<ItemLineAdd>
<ItemRef>
<ListID>30000012-1247123980</ListID>
</ItemRef>
<Desc>My Item Description</Desc>
<Quantity>1</Quantity>
</ItemLineAdd>
</ItemReceiptAdd>
</ItemReceiptAddRq>
</QBXMLMsgsRq>
</QBXML>