How to run select statement with a list of where value SQL 2008?

I have a list of PO numbers that is in a text file:

textfile.txt
12323
53221
23953
20391
10393
etc.
etc..

Here is the statement I am running

select dbo.PurchaseOrder.SiteID from dbo.PurchaseOrder where dbo.PurchaseOrder.PONumber = '12323'

This statement will give me the output of the value. But how I make it so it run all the PO number in the textfile.txt file without me having to run it one by one.

Thanks in advance..