You could do this with the smart lighting app. Motion+ outlet = on. However, why not get a Siren that integrates into ST. Such as…
This will integrate into SHM. Has a builtin backup battery. So you would have your motion sensor and siren added to SHM. There is an option to dictate the duration of the siren as well in SHM. This seems to be the better approach. Then use the extra outlet to turn on a lamp!