Page Events
1. In the 'Properties for Page' pane at the right, select the 'Event Type' as 'Webservice' for the required event (On Load, Before Save/Insert, Before Save/Update, After Save/Insert, or After Save/Update).
2. In the 'Method' text box, enter the Apex web service name as ClassName.MethodName (for example, SVMXC__INTF_WebServicesDef.INTF_WORD_USG_GetPrice_WS).