Solution was easier than I thought. 

Set key = "secretkey"
Set message = "messageinabottle"
Set secretByteArray = ##class(%SYSTEM.Encryption).Base64Decode(key)
Set signatureBytes = $SYSTEM.Encryption.HMACSHA(256, message, secretByteArray)

Set requestSignatureBase64String = ##class(%SYSTEM.Encryption).Base64Encode(signatureBytes)