set default button in asp.net

<form id="form1" runat="server" DefaultButton="btnLogin">

 <asp:TextBox ID="txtUserName" runat="server" ToolTip="Enter Your User Name"
              TabIndex="1"  placeholder="User Name"></asp:TextBox>

      <asp:TextBox ID="txtpassword" runat="server" ToolTip="Enter Your PassWord Name"
                               TabIndex="2" placeholder="Password" TextMode="Password"></asp:TextBox>

 <asp:Button ID="btnLogin" runat="server" Text="Sign In"  TabIndex="3"
                                                   OnClientClick="return Validate()" OnClick="btnLogin_Click" />

  </form>