This article explains how to include files in your .aspx file. Look at the comments below:

The easier way is create a new file of type "Web User Control" then drag and drop the file in the Design View of your page.

It will create the following code in your page:

//in the beggining of file
<%@ Register Src="includefile.ascx" TagName="TAGNAME" TagPrefix="tagprefix" %>

//in the place you need to include the file
<tagprefix:TAGNAME ID="tagid" runat="server" />