Private Sub Form1_Shown(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Shown
        ContextMenuStrip1.Show(New Point(100, 200))
    End Sub

Public Class Form1
    Dim CustomContextMenu1 As New CustomContextMenu
    Private Sub Form1_Shown(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Shown
        CustomContextMenu1.Show()
    End Sub
End Class
Public Class CustomContextMenu
    Inherits ContextMenuStrip
    Public Overloads Sub Show()
        Me.Show(New Point(100, 200))
    End Sub
    Public Sub New()
        Me.Items.Add("Открыть")
        Me.Items.Add("Сохранить")
    End Sub
End Class