Retrieve DataExtensionField Object Id

<?xml version="1.0" encoding="UTF-8"?>
<s:Envelope xmlns:s="http://www.w3.org/2003/05/soap-envelope" xmlns:a="http://schemas.xmlsoap.org/ws/2004/08/addressing" xmlns:u="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-utility-1.0.xsd">
   <s:Header>
      <fueloauth xmlns="http://exacttarget.com">[access_token_goes_here]</fueloauth>
   </s:Header>
   <s:Body xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
      <RetrieveRequestMsg xmlns="http://exacttarget.com/wsdl/partnerAPI">
         <RetrieveRequest>
            <ObjectType>DataExtensionField</ObjectType>
            <Properties>Name</Properties>
            <Properties>ObjectID</Properties>
            <Filter xmlns:ns1="http://exacttarget.com/wsdl/partnerAPI" xsi:type="ns1:SimpleFilterPart">
               <Property>DataExtension.CustomerKey</Property>
               <SimpleOperator>equals</SimpleOperator>
               <Value>[Data_Extension_External_Key_Goes_Here]</Value>
              </Filter>
         </RetrieveRequest>
      </RetrieveRequestMsg>
   </s:Body>
</s:Envelope>