Use XmlPoke to replace Xml node

My goal was to update the project file (.proj) of a Visual Studio 2008 project in NAnt script. At first I thought it was a simple task, and I found all the knowledge I need to learn and figure out the proper XPath to find the path I need to modify:

/Project//PostBuildEvent

However, I couldn't get the script XmlPoke running. After a while, I realize that the code WILL run if I take out the name space reference in the project file:

xmlns="http://schemas.microsoft.com/developer/msbuild/2003"

I looked into Nant's manual, and found out how to specify namespace entry. But I have problem figuring out what namespace prefix I should use.

Finally I came across this post, which gave me the exact way to write it. So here it goes:

    <target name="SetEnviornmentForBeforeCompilation">
       <xmlpoke
          file="Configuration.csproj"
          xpath="/ns:Project//ns:PostBuildEvent"
          value="$(TargetPath) $(SolutionDir) BUILD" >
             <namespaces>
                   <namespace prefix="ns" uri="http://schemas.microsoft.com/developer/msbuild/2003" />
             </namespaces>
        </xmlpoke>
   </target>