Without knowing the 3rd party platform and it's expectations for the content of the message, the best I can offer are some examples.
This is probably the most basic example of computing an HMAC256 hash for a signature:
private string GetHMAC(string text, string key)
key = key ?? "";
using (var hmacsha256 = new HMACSHA256(Encoding.UTF8.GetBytes(key)))
var hash = hmacsha256.ComputeHash(Encoding.UTF8.GetBytes(text));
You'll need to add a reference to the assembly mscorlib.dll
and the namespace System.Security.Cryptography
in your Code Options
If you grab a copy of the Blue Prism AWS REST Utility
VBO from the Digital Exchange, there's a more complex example of how to calculate an HMAC256 signature for requests to AWS services. This is an example where the platform (AWS in this case) has specific expectations regarding the various bits of data that should be included in the signature.
There are some additional examples on this Stack Overflow