在下拉菜单的 onchange 事件中添加您的客户端函数名称，如下所示:

Add your client side function name in onchange events of dropdown like below :

<asp:DropDownList ID="DropDownSubject" runat="server" DataTextField="Word" 
      AutoPostBack="True" OnSelectedIndexChanged="SelectedChange" 
      onchange="changeCursor()">
</asp:DropDownList>