You can probably look at Kim Jong Un's crossing of the South Korea border during Trump's admin
You have that back-to-front
It was Trump who crossed the border, not Kim.
As far as I remember, Kim Jong Un briefly stepped in to South Korea, after the RoK president urged him to do so.
https://www.youtube.com/watch?v=1IyLMz4buesThis one happened back in 2018, when Kim Jong Un of DPRK and Moon-Jae-in of Republic of Korea met each other in Panmunjom in the Demilitarized Zone (DMZ). But things went bad after that and the relations between these two countries are now at a new low. Moon-Jae-in himself benefited greatly from improved relations back in 2018, and his party managed to win the parliamentary elections in 2020.