If you just want to fire the OnSelectedIndexChanged, this is how it should look:
Page.aspx - Source
<FooterTemplate> <asp:DropDownList ID="ddlOptions" runat="server" AutoPostBack="true" onselectedindexchanged="ddlOptions_SelectedIndexChanged"> <asp:ListItem>Option1</asp:ListItem> <asp:ListItem>Option2</asp:ListItem> </asp:DropDownList> </FooterTemplate>
Page.aspx.cs - Code-behind
protected void ddlOptions_SelectedIndexChanged(object sender, EventArgs e) { //Event Code here. }

And that's it. Your event will be called now.

If you just want to fire the OnSelectedIndexChanged, this is how it should look:
**Page.aspx - Source**
<FooterTemplate> <asp:DropDownList ID="ddlOptions" runat="server" AutoPostBack="true" onselectedindexchanged="ddlOptions_SelectedIndexChanged"> <asp:ListItem>Option1</asp:ListItem> <asp:ListItem>Option2</asp:ListItem> </asp:DropDownList> </FooterTemplate>

**Page.aspx.cs - Code-behind** protected void ddlOptions_SelectedIndexChanged(object sender, EventArgs e) { //Event Code here. }
And that's it. Nothing more is needed.