I noticed you were able to attain the loading screen in the event where God-King Garen and God-King Darius were promoted, but I just purchased God-King Garen now and was wondering if I could get the loading screen now.
Alas no, that was a limited time thing for people who participated in the event. It was designed to distinguish those from the event from those who purchased the skin later. Some games have limited edition skins, but thankfully Riot just prefers to make it where those who were part of an actual event get a special loading screen while still letting the skin be purchased later for everyone else.
In the off case you bought said border with tokens during the event without the skin, it should still go through.
Outside of that, it's impossible.
