Several things:
1) It may be urgent to you, but it isn't to us. All that your stressing the urgency does is to make us think you have left it too late, and want us to do it for you. This annoys some people, and can slow a response.
2) Never, ever, accept code from a insecure website to handle anything to do with real money.
You do not know who is giving you the code, you do not know what it does, you do not know that it places the monies correctly into the appropriate account, without passing the details to any third parties.
Only get such code from reputable card transaction service companies - the scope for fraud otherwise is far too large. And remember, you personally could be liable for any monies lost if your action is seen to be negligent - which getting your code from a public forum would most certainly be!
3) Upgrade from .NET 1.1 to at least V4, and it's supported:
HMACSHA256 Class (System.Security.Cryptography) | Microsoft Docs[
^] using outdated frameworks for security apps? Not a good idea, not at all...