538 seems to have acquired their numbers for all candidates down to 1%. Just scroll down to Nov. 20, 2019 (or search for "Swayable") and it will show the whole list of polls, among both adults and likely voters. Normally 538 includes those who polled at zero percent, so it's odd that they are not listed here. It would make more sense with a open-ended question (which has a potential for infinite candidates at zero), but they make it explicit in their methodology section that the candidates were read from a list:
Each respondent was presented with a randomized list of all Democratic and Republican candidates from which to make selections.
