We are going to use the extrinsic elements.
<ItemIn quantity="1">
<ItemDetail>
<UnitPrice>
<Money currency="USD">1.23</Money>
</UnitPrice>
<Description xml:lang="en-US">product 123</Description>
<UnitOfMeasure>FT</UnitOfMeasure>
<Classification domain="UNSPSC">31151601</Classification>
<ManufacturerPartID>1234</ManufacturerPartID>
<ManufacturerName>Test Company</ManufacturerName>
<Extrinsic name="QuoteId">12345</Extrinsic>
<Extrinsic name="QuoteLine">1</Extrinsic>
</ItemDetail>
</ItemIn>