I am a US citizen & I have a L Visa. How to get a medical certificate to board to China from Korea.

I am a US citizen & I have a L Chinese Visa, I will be in Korea from July to September, then I would like to visit China in September. Please tell me what is requirements to get a medical certificate to board a flight to China. I finished & completed COVID-19 Vaccination Moderna !st & 2nd dose on April 10,2021. I would like to know where to get the necessary test and summit the result in Korea in order to get health code.
Thanks & best regards,
Richard