Add <Property Id="WIXUI_EXITDIALOGOPTIONALCHECKBOX" Value="1" />
to give the checkbox property its "checked" value.
It goes outside the UI
element. Here's a complete example:
<UI>
<UIRef Id="WixUI_Minimal"/>
<Publish Dialog="ExitDialog"
Control="Finish"
Event="DoAction"
Value="LaunchApplication">WIXUI_EXITDIALOGOPTIONALCHECKBOX = 1 and NOT Installed
</Publish>
</UI>
<Property Id="WIXUI_EXITDIALOGOPTIONALCHECKBOX" Value="1"/>