mi correggo:
questa sintassi funziona
codice:
    Private Sub RadioButtonA_Init(ByVal sender As Object, ByVal e As System.EventArgs) Handles RadioButtonA.Init
        Dim r As System.Web.UI.WebControls.RadioButton = DirectCast(sender, System.Web.UI.WebControls.RadioButton)
        r.Attributes.Add("onclick", "r_ab_click();")

    End Sub