If you're interested in buying new construction, the builder's agent on site will be ready to help you with the process. And, even if it seems like plug and play to sign up on-site make no mistake:
The builders agent will always have the builders best interest in mind.
You only have one chance to bring in a realtor to represent negotiate and fight for you and that is from the get-go. Sometimes a year long of potential errors, negotiations and chances for you to get more. Feel free to call any builder site before going in and ask them, "if I come in without a realtor can I get that commission to lower the price?".
Sellers (including builders) already pull all representation fees out before pricing the property as a marketing expense. Realtor.com article explains who pays what how much to continue reading.