If we want to have a KO tourney, then there 'has' to be a buyin:

Thank you for your e-mail. In order to have a knockout bounty in your
tournament, there needs to be a tournament buy-in, as a percentage of the
tournament buy-in is used as the knockout bounty.

Please be advised that the minimum juice for private knockout tournaments
is presently $0.50 and therefore the minimum buy-in would be $1+$0.50.