Private Sub btkhinzufuegen_Click(sender As Object, e As EventArgs) Handles _
  btkhinzufuegen.Click
        KundenHinzufuegen.Show()
        Me.Hide()
End Sub

Private Sub btkhinzufuegen_Click(sender As Object, e As EventArgs) Handles _
  btkhinzufuegen.Click
        Hauptform.Show()
        Me.Hide()
End Sub

Hautpform.show
unload me

EALA FREYA FRESENA

Public Class frmHauptForm
 
    Dim WithEvents frmKH As New frmKundenHinzufügen
 
    Dim WithEvents btn As New Button With _
       {.Parent = Me, .Text = "Kunden hinzufügen", .width =200}
 
 
    Private Sub DoSomething()
        '......
        MsgBox("Something should be done")
    End Sub
 
 
    Private Sub frmHauptForm_Load(sender As System.Object, _
        e As System.EventArgs) Handles MyBase.Load
        DoSomething()
    End Sub
 
 
    Private Sub btn_Click(sender As Object, _
        e As System.EventArgs) Handles btn.Click
 
        frmKH.Show()
        Me.Hide()
    End Sub
 
 
    Private Sub frmKH_ShowHauptform(sender As Object, _
                     e As System.EventArgs) Handles frmKH.ShowHauptform
 
        DoSomething()
        Me.Show()
    End Sub
End Class
 
 
Public Class frmKundenHinzufügen
 
    Dim WithEvents btn As New Button With _
        {.Parent = Me, .Text = "Zurück zur Hauptform", .Width = 200}
 
    Public Event ShowHauptform(sender As Object, e As EventArgs)
 
 
    Private Sub btn_Click(sender As Object, _
                    e As System.EventArgs) Handles btn.Click
 
        Me.Hide()
        RaiseEvent ShowHauptform(Me, EventArgs.Empty)
 
    End Sub
End Class