Posting data to a Web service is now possible through auto-generated forms of request modeler or custom forms. Hold your thought for custom forms as now the data captured in custom forms can also be posted to a Web service. You just need to configure the data source accordingly in the Connections module in Studio and create a request using this data source in Request Modeler. Then map the Request Model ID to the custom forms in Form Designer and you are done!
Figure 
Figure 6: Request Creation Screen Showing Web Services Option
For more information, refer Posting Data using the POST Method.