:
<Component Id="EventLogRegKeys" Guid="{my guid}">
<RegistryKey Id="Registry_EventLog" Root="HKLM" Key="SYSTEM\CurrentControlSet\Services\Eventlog\Application\MyApp" Action="create">
<RegistryValue Id="Registry_EventLog_EventSourceDll" Action="write" KeyPath="yes" Name="EventMessageFile" Type="string" Value="C:\Path\To\File.dll" />
</RegistryKey>
</Component>