Connection in Beijing to Guangzhou 144 hours privilege

I am American. My plane flies from the USA with a connection in Beijing to Guangzhou. Then from Guangzhou I will continue my journey to Bangkok and then back to the USA. At which airport should I declare my intention to stay in Guangzhou 144 hours (in Beijing where I will have a transfer in Guangzhou or directly at Guangzhou airport). I do not plan to leave the airport in Beijing and enter the city in Beijing. This privilege I would like to use for Guangzhou City