Hi kankon,
Use string.Format with place holder to replace with TextBox value.
HTML
<asp:TextBox runat="server" ID="txtNumber" />
Code
C#
string url = string.Format("https://www.aspforums.net/Threads/{0}/Insert", txtNumber.text);
System.Diagnostics.Process.Start(url);
VB.Net
Dim url As String = String.Format("https://www.aspforums.net/Threads/{0}/Insert", txtNumber.text)
System.Diagnostics.Process.Start(url)