I'm no expert on sending web requests, but this worked on a system that I have access to. In the example below, I created a brand new blank project, added a button to the form. Try this:
Imports System.Net
Public Class Form1
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Dim url As String = "http://192.168.1.104:3480/data_request?id=lu_action&output_format=json&DeviceNum=18&serviceId=urn:upnp-org:serviceId:SwitchPower1&action=SetTarget&newTargetValue=1"
Using client As New WebClient
Dim result = client.DownloadString(url)
MessageBox.Show(result)
End Using
End Sub
End Class