Here is the information for the Seoul Dongdaemoon testing center. The site is a Prometric Test Center, which provides testing services to a multitude of companies, including ETS.
ADDRESS:
HSF Inc. 6th Floor,
Sungwoo Building 324, Toegye-ro, Jung-gu
PHONE NUMBER: 02-318-0012
They have English-speaking staff that is very helpful.
As of November 2021, there is no vaccination or PCR test required. However, a mask is required at all times during testing.