是on Tik Tok
You have to say -on TikTok.
TikTok is, as well other social medias, a digital platform, in other words, it's something that has a surface to roll through.
If you use -in it gets implied you get into the platform.
The same rule works for Facebook/Instagram/YouTube and so forth.
in和on的區別如下：
1、In有3種空間的概念
一種是在一個比較封閉的立體空間中。
如：in a room, in a building, In a shop, in a car
另一種用法是在一個平面的地方里。
如：in a garden, in a town, in the city centre, in China。
第三種是用于在有身體接觸的情況下，一般是液體里。
如：in a pool, in the sea, in a river.
2、On也有兩種空間的解釋
一種是在某一個水平的平面、緊貼著的上面，
比如on the shelf, on a plate, on the bal買粉絲ny, on the floor.
還有一種不一定是水平的，但也是有接觸面的，
比如on the wall, on the door, on the ceiling.