Appending comma in given number

Please refer this post, maybe you need to tweak a logic a bit but majority of the portion or say core logic you can grab it from there:

https://social.msdn.microsoft.com/Forums/en-US/c379b552-3ae4-4466-8bba-b8cacb022247/how-to-get-digits-from-a-single-long-type-variable?forum=csharpgeneral