OPENIDRELYINGPARTY:
OpenIdRelyingParty Openid = new OpenIdRelyingParty();
var response = Openid.GetResponse();
if (response == null)
{
  try
  {
    const string providerUrl = "mioprovider";
    IAuthenticationRequest request = Openid.CreateRequest(providerUrl);
    
    // voglio inserire un valore da passare al provider
    
    return request.RedirectingResponse.AsActionResult();
  }
  catch (ProtocolException ex)
  {
    // gestione errori
  }
}
else
{
  // risposta dal provider
}

PROVIDER:
var OpenIdProvider = new DotNetOpenAuth.OpenId.Provider.OpenIdProvider();
IRequest request = OpenIdProvider.GetRequest();
if (request != null)
{
  // Some requests are automatically handled by DotNetOpenAuth.  If this is one, go ahead and let it go.
  if (request.IsResponseReady)
  {
    return OpenIdProvider.PrepareResponse(request).AsActionResult();
  }
  
  // estraggo l'informazione che ho passato dall'OpenIdRelyingParty
  
  // This is apparently one that the host (the web site itself) has to respond to.
  ProviderEndpoint.PendingRequest = (IHostProcessedRequest)request;

  return RedirectToAction("Login", "Account", new { returnUrl = Url.Action("ProcessAuthRequest") });
}