OnClient Click javascript using Freetextbox

My javascript doesnt detect my function using Freetextbox..

here's my code in javascript: I don't know if is the right code for Freetextbox.

function validateFreetextbox()
{

    if(document.FTB_API["txt_Decs"].GetHtml == ''  )   
    {
        alert("Please Fill up desciption.");
        return false;
    }
    else
    {
        return true;
    }
}

here'开发者_如何学Pythons the code in my asp.net/C# button.

 <asp:Button ID="btn_Add" runat="server" OnClick="btn_Add_Click" Text="Add" 

 Width="231px" Font-Size="11px" Font-Bold="false" ForeColor="#333333"  

 onclientclick=" validateFreetextbox()" />

---

onclientclick=" validateFreetextbox()" /> should be OnClientClick="return validateFreetextbox()" />