How Should I compute signatures in API? - vb.net

Question

I want to create API for yobit. For now, I will do something very simple, namely get info.
In Python, the code sort of looks like this
url = 'https://yobit.net/tapi'
values['method'] = method
values['nonce'] = str(int(time.time()))
body = urlencode(values)
signature = hmac.new(self.secret, body, hashlib.sha512).hexdigest()
headers = {
'Content-Type': 'application/x-www-form-urlencoded',
'Key': self.key,
'Sign': signature
}
I basically want to replace code hmac.new(self.secret, body, hashlib.sha512).hexdigest() with vb.net equivalent.
This my current code
Public Overrides Sub readbalances()
Dim secret = _secret
Dim url = "https://yobit.net/tapi"
Dim key = New Tuple(Of String, String)("Key", _apiKey)
Dim parameters = "method=getInfo&nonce=" + Str(Int(DateTime.Now))
Dim sig = New System.Security.Cryptography.HMACSHA512().ToString 'What should I put here?
Dim sigtupple = New Tuple(Of String, String)("Sign", sig)
Dim result = CookieAwareWebClient.downloadString1("url", {key, sigtupple}, parameters)
End Sub
What should I do?
Content of downloadString1 is the following but let's just say it performs as expected
Public Shared Function downloadString1(url As String, headers As Tuple(Of String, String)(), postdata As String) As String
Dim wc = New CookieAwareWebClient
For Each header In headers
wc.Headers.Add(header.Item1, header.Item2)
Next
Dim result = wc.UploadString(url, postdata)
Return result
End Function