https://www.nber.org/system/files/working_papers/w29053/w290...

First names: "We draw racially distinctive first names from two sources. First, we use the same set of names in Bertrand and Mullainathan (2004), which are in turn drawn from Massachusetts birth records covering 1974 to 1979. Second, we supplement with names drawn from administrative records on speeding infractions and arrests provided by the North Carolina Administrative Office of the Courts and covering 2006 to 2018. We pick the most common names among drivers born between 1974 and 1979 with race- and gender-specific shares of at least 90%"

Last names: "Last names are drawn from 2010 Decennial Census data. We use the names with highest race-specific shares that occur at least 10,000 times, picking 26 total for each race group"

They probably used this same method to assume an answer.