windows messaging with a service-application

Is there a possibility to use Windows-Messaging-Routines
in services?

I am currently writing a service-application (TServiceApplication),
and in my services I want to interact with "the world out there"
using Windows-Messaging.

How can this be done?
If no, how *does* a service-application interact with other
programs then?

I ask because I couldn't find any HWND-Handle to use for

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