ToolbarInsertLinkDialogButton Class

protected void Page_Load(object sender, EventArgs e)
{
    HtmlEditorToolbar stToolbar1 = HtmlEditorToolbar.CreateStandardToolbar1();
    ToolbarInsertLinkDialogButton linkButton = new ToolbarInsertLinkDialogButton();
    stToolbar1.Items.Add(linkButton);
    ASPxHtmlEditor1.Toolbars.Add(stToolbar1);
}

<dx:ASPxHtmlEditor ID="ASPxHtmlEditor1" runat="server">
    <Toolbars>
        <dx:HtmlEditorToolbar Name="StandardToolbar1">
            <Items>
                        <dx:ToolbarInsertLinkDialogButton>
                        </dx:ToolbarInsertLinkDialogButton>
            </Items>
        </dx:HtmlEditorToolbar>
    </Toolbars>
</dx:ASPxHtmlEditor>