Workflow, API: text/xml

I'm trying to connect via api to add a subscriber to my ESP;

I've setup a resource, but also tried using js to webhook the variables below to convert to xml....

<ApiRequest xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance' xmlns:xs='http://www.w3.org/2001/XMLSchema'>
<ApiKey>$APIKEY</ApiKey>
       <Data xsi:type='Subscriber'>

       <Mode>AddAndUpdate</Mode>

       <Force>false</Force>

       <ListId>$LIST</ListId>

       <Email>$EMAIL</Email>

       <Firstname></Firstname>

       <Lastname></Lastname>

       <TrackingCode></TrackingCode>

       <Vendor>$CAMPAIGN</Vendor>

       <Ip>$IP</Ip>

       <Properties>

          <Property>

             <Id>$custom_field</Id>

             <Value xsi:type='xs:string'>$CAMPAIGN</Value>

          </Property>

       </Properties>

    </Data>

 </ApiRequest>

for the life of me, I'm not sure how to do this in a workflow