How to append (;) to a string in a autocomplete text box
string[] result =  txt_to.Text.Split(';');

<asp:AutoCompleteExtender ID="AutoCompleteExtender1" runat="server" TargetControlID="txt_to"
                            MinimumPrefixLength="2" CompletionInterval="10" CompletionSetCount="3" EnableCaching="true"
                            FirstRowSelected="true" UseContextKey="True" ServiceMethod="GetCompletionList"
                            DelimiterCharacters=";" ShowOnlyCurrentWordInCompletionListItem="true" />

function GetKeyPress(){
    if (window.event.keyCode == 13){
        document.getElementById ('txt_to').value +=";";
    }
}


 function addSemiColumn(){
     document.getElementById ('txt_to').value +=";";
}

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
    If IsPostBack Then
        txt_to.Focus()
    End If
    txt_to.Attributes.Add("OnKeyPress", "GetKeyPress()")
End Sub