I know that a solution has been posted above but thought that my 5 cents worth might help future browsers on the question asked.
I also know that 'HttpClient' is generally considered the newer and more modern option for making HTTP requests in .NET but I have found that 'WebClient' has been around for a longer time and is part of the System.Net namespace. It is simpler and easier to use for basic scenarios but lacks some of the advanced features and flexibility offered by 'HttpClient'. Based on the complexity of your usage, this might be the easier way out if you don't need the more advanced features so if you're looking for a simpler alternative, you might consider using the 'WebClient' class, which is more straightforward for basic scenarios (Tongue in cheek here as many might disagree with me).
To use 'WebClient, you can try the following -
Imports System.Net
Public Class Form1
Dim webClient As New WebClient()
Dim baseUrl As String = "http://192.168.178.201/relay/"
Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click
SendRequest("0?turn=off")
End Sub
Private Sub Button2_Click(sender As System.Object, e As System.EventArgs) Handles Button2.Click
SendRequest("0?turn=on")
End Sub
Private Sub Form1_FormClosing(sender As Object, e As FormClosingEventArgs) Handles MyBase.FormClosing
webClient.Dispose()
End Sub
Private Sub SendRequest(endpoint As String)
Try
Dim result As String = webClient.DownloadString(baseUrl + endpoint)
Catch ex As Exception
End Try
End Sub
End Class