In .aspx Page
 
txtDemo" runat="server" TextMode="MultiLine" >
 
txtDemo_AutoCompleteExtender" runat="server"
                             TargetControlID="txtDemo" Enabled="True" 
                             EnableCaching="true" MinimumPrefixLength="3"  
                             FirstRowSelected="True"  ServiceMethod = "GetEmployeeName"    
                             ServicePath = "EmployeeName.asmx">
In code behind
 
protected void Page_Load(object sender, EventArgs e)
{    
  txtDemo_AutoCompleteExtender.DelimiterCharacters = "\r\n";
}