You really can't book a lap child (if that's what you are trying to do) for an international flight online. You need to call, and they will tell you what it costs for the infant.

From the FAQ's

How do I book a seat for an infant?
There are two options when traveling with an infant (under 2 years of age):

Infant traveling in an adult's lap

Within the U.S. or between the U.S. and Canada:
Although infants are not required to occupy a seat, your reservation must be documented as including an infant. If you decide to fly with an infant in your lap, please book your travel on united.com and then contact United Reservations to advise that an infant will be traveling on your itinerary.

Between the U.S. and Puerto Rico or the U.S. Virgin Islands:
The infant fare is equal to 10% of the applicable adult fare. Please contact United Reservations to purchase a ticket for an infant who will be traveling in your lap.

International:
An infant is required to have a ticket in order to travel as a lap child. Infant discounts may be offered based on your destination. Please contact United Reservations to inquire about infant discounts or to purchase an international ticket for your infant.

Infant occupying a seat

Within the U.S. or between the U.S. and Canada, Puerto Rico or the U.S. Virgin Islands:
You may purchase a separate seat for your infant for increased comfort. Infants occupying a separate seat are subject to the full adult fare. To book a separate seat for your infant, please contact United Reservations.

International:
Infant discounts may be offered based on your destination. Contact United Reservations to inquire about infant discounts or to purchase an international ticket for your infant.