Wix: Show conditional message box

For example:

Scenario:

Show message box only during installation and MYPROPERTY has to be equal to "TestValue".

1> Define custom action

 <CustomAction Id="POPMSG " Script="vbscript">

      <![CDATA[MsgBox("For Custom Action POPMSG.")]]>

 </CustomAction>

2> Put this custom action inside <InstallExecuteSequence></InstallExecuteSequence> or <InstallUISequence></InstallUISequence>

<Custom Action="POPMSG" Before="CostInitialize" >

   <![CDATA[MYPROPERTY="TestValue" AND NOT Installed]]>

</Custom>

Reference:

http://stackoverflow.com/questions/8239188/wix-show-conditional-message-box-without-cancel