How do I see device account number?

Amex email about using Apple Pay reads:

"If a merchant asks for the last 5 digits of your Card after you've made a purchase with your American Express Card in Apple Pay, please use the last 5 digits of the Device Account Number. ... To see the Device Account Number for your Watch, visit the Passbook section of the Apple Watch app."

If I go to Passbook app on the Watch, how do I see the Device Account Number? It seems the only option at that point is to pay or not.