How to pass a request parameter or attribute to a portlet depends on the portal. You have to consult the user's guide of your favorite portal for details. The following is an example that uses Potix Portal.

<layout contentType="text/html">
    <title>ZK Porlet Demo</title>    
    <header name="Cache-Control" value="no-cache"/>    
    <header name="Pragma" value="no-cache"/>    

    <vbox>    
        <hbox>        
            <servlet page="sample1.zul"/>            
            <portlet name="zkdemo.zkLoader">            
                <attribute name="zk_page" value="/test/sample2.zul"/>                
            </portlet>            
        </hbox>        
    </vbox>    

    <molds uri="~./pxp/html/molds.xml"/>    
</layout>