Authorize.net issues with American Express

Does anyone know how to get amex authorized with authorize.net? I have authorize set up correctly. On the site it mentions "Acce开发者_如何学Pythonpted Payment Method : American Express, Discover, MasterCard, Visa" because I have an underwriter and everything setup for amex.

I ask this on stack overflow, because its a question about what are required fields for the form input. Other programmers must have run into this before. I can't seem to isolate the problem.

I called and it turns out you need to have your underwriter in place and you also need a seperate agreement with American Expess, sign up at https://www209.americanexpress.com/merchant/marketing-data/pages/home?inav=merch_home2