I could solve it but I forget to post my solution maybe is useful for someone:

I've use MarkItUp to get this out, this is my usercontrol:

<asp:TextBox ID="TextBox1" TextMode="MultiLine" CssClass="markItUp" runat="server"></asp:TextBox>
    <!-- markItUp! -->
    <script type="text/javascript" src="/scripts/markitup/jquery.markitup.js"></script>
    <!-- markItUp! toolbar settings -->
    <script type="text/javascript" src="/scripts/markitup/sets/markdown/set.js"></script>

<script type="text/javascript">
    $(function () {
        $('.markItUp').markItUp(mySettings);
    });
</script>

and this is the .acsx.cs file :

 protected override void OnInit(EventArgs e)
         {
             var skin = new Literal { Text = "<link href=\"/scripts/markitup/skins/simple/style.css\" type=\"text/css\" rel=\"stylesheet\" />" };            
             var set = new Literal { Text = "<link href=\"/scripts/markitup/sets/markdown/style.css\" type=\"text/css\" rel=\"stylesheet\" />" };
            
             Page.Header.Controls.Add(skin);
             Page.Header.Controls.Add(set);
         }
 
         public object value
         {
             get
             {
                 return TextBox1.Text;
             }
             set
             {
                 if (value != null)
                 {
                     TextBox1.Text = value.ToString();
                 }
             }

Thanks.