On WiX 3.8, the only way I found this is
<CustomAction Id="LaunchFile" FileKey="..." ExeCommand="" Return="asyncNoWait" />
<InstallExecuteSequence>
<Custom Action="LaunchFile" After="InstallFinalize">NOT Installed</Custom>
</InstallExecuteSequence>
which runs after the Install button in the user interface and before the Finish button. Also works great in mode /quiet.
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