There is no requirement, or even custom, for presidential candidates to disclose a list of potential Supreme Court nominees they would appoint while in office. Donald Trump's 2016 list of potential nominees was unusual, and followed the Republican-controlled Senate's move to hold open the seat vacated by the early 2016 death of Justice Antonin Scalia until after the 2016 election.
The list was compiled for Trump by two conservative organizations, and helped the candidate appeal to voters who were initially wary of his commitment to key conservative goals.