Write to Eventlog in an Inno Setup Script

1. Create a simple console Application, which is capable to write to the Windows Event Log. It should expect the Versionnumber as a "Commandline Paramter", which you can read with ParamStr(1). There is an Example how to do this here.

2. Install ISPP (Inno Setup Pre Processor). This can be done very easy with the "Inno Setup QuickStart Pack"

3. Read the Version Infomation in the Inno Setup Script (at the top) from the desired Application with GetFileVersion into a variable.
Example:
define #AVersionNumber GetFileVersion("..\MyApp.exe")

4. Call your Application, which writes to the Eventlog in the [Run] Section of the Inno Setup Script with the Version Number as the Parameter.
Example:
[Run]
Filename: "{app}MyApplication.exe"; Parameters: {#AVersionNumber}