Web Item link dynamic

[Pedro Morocho]

I am creating a button with the example from the jira server documentation

  <atlassian-plugin name="Hello World Plugin" key="example.plugin.helloworld" plugins-version="2"> 
<plugin-info> 
<description>A basic web item module test</description> 
<vendor name="Atlassian Software Systems" url="http://www.atlassian.com"/> <version>1.0</version> 
</plugin-info> <web-item key="google_home" name="Google Home" section="system.header/left" weight="60">
 <description key="item.google.home.desc">Simple link to google.com.</description>
 <label key="item.google.home.label" /> 
<link linkId="google_home">
http://google.com/${projectID}</link>
 </web-item> 
</atlassian-plugin>

How can I pass a parameter to the url and what classes should I create?