I booked a meal in the US Mission for tomorrow. How can I let the merchant know that I have made a reservation?

Generally speaking, the US delegation will eat on that day. Please see if there is a time limit for your order. Some can't be used on weekends. If the merchant says that he needs to make an appointment one day or one hour in advance, he can call in advance to make an appointment. If not, go directly. When you go, text your order to the clerk at the front desk and he will help you.