I was able to calculate the same hash using the rawBody using Nethereum's Sha3Keccack. The Sha3 function used by web3 (and thus Moralis) is Keccack256, not the same SHA3 used by .NET's System.Security.Cryptography. The implementation looks like:

var hashString = Sha3Keccack.Current.CalculateHash(rawBody + "<streams secret>");