RNGCryptoServiceProvider is obsolete

You can use RandomNumberGenerator like this:

var randomNumber = new byte[32];
string refreshToken = "";

using (var rng = RandomNumberGenerator.Create())
{
     rng.GetBytes(randomNumber);
     refreshToken = Convert.ToBase64String(randomNumber);
}

Example of result: fHu4SOyv03PhHvqWx5WGmI5xQFnH00CmLCV3IYJd5Dw=

Just adjust this to your case.