Try setting your postdata to be a string like "uid=" + uid + "&userid=" + userid + "&password=" + password etc
i.e. '&' between different fields and '=' between the field name and value.
Alternatively use FormUrlEncodedContent
Something like
Dim params = New Dictionary(Of String, String)
params.Add("uid", uid)
params.Add("userid", userid)
params.Add("password", password)
....
Dim postdata= New FormUrlEncodedContent(params)