Operations under /Library/Operations/WS-Management in a Base CP currently do not provide inputs for http proxy settings and if you would like to call one of the operations behind an http proxy server, try one of the following options.
1) Use the Web Services Wizard(ws-wizard.bat) under studio/tools and create an OO flow that calls a web service that is equivalent to an operation under /Library/Operations/WS-Management. You are able to configure proxy settings in the wizard GUI.
2) Try setting generic system properties for JVM regarding an http proxy server. Such system properties can be set in wrapper.conf for Central and RAS.
For example, add the following two lines in central-wrapper.conf: