If you are using Modeler version 6.5 or before, then you must specify the input into the Webservice step in the previous step. The webservice step in Modeler 6.5 does not generate into an asset on the Integration Server.
In BPM 7.0, the Webservice Step has been replaced with Webservice connectors that are being generated on the Integration Server. I would suggest that is the easier approach. Create a service from a WSDL on the Integration Server and orchestrate that as part of your business process.
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