As per my understanding; you want some popup to shown as alert on your Mashup. You can use Status message widget for that.
Add your custom service in your Mashup with OnLoad checked so that service is invoked as soon as Mashup is loaded.
Also, bind the output of the Service to the message property of the Status widget.
Now on ServiceInvokeCompleted of this service (available in the right bottom after clicking on the custom service) bind the showMessage event of the Status widget.
So, as soon as the Mashup is loaded you will get a popup with the alert.
I hope it helps.