<div class='col-sm-6'>
  <asp:PlaceHolder ID="PlaceHolder1" runat="server"></asp:PlaceHolder>
</div>
<br />
<asp:Label ID="Label1" runat="server" Text="" EnableViewState="False"></asp:Label>

Option Strict On

Partial Class prove_a
    Inherits System.Web.UI.Page


    Protected Sub Page_Load(sender As Object, e As System.EventArgs) Handles Me.Load
        Put_Attivita()
    End Sub

    Private Sub Put_Attivita()
        Dim c As New CheckBox()
        With c
            .ID = "mioId"
            .Text = "Scalate Monti"
            .TextAlign = TextAlign.Left
            .AutoPostBack = True
        End With
        AddHandler c.CheckedChanged, AddressOf mioIdCheckChanged
        Me.PlaceHolder1.Controls.Add(c)
    End Sub

    Private Sub mioIdCheckChanged(sender As Object, e As EventArgs)
        Dim c As CheckBox = DirectCast(sender, CheckBox)
        Me.Label1.Text = String.Format("{0}.checked = {1}", c.ID, c.Checked)
    End Sub

End Class