‘On’ is most natural. We think of lists as being on paper that we write on. But ‘in’ could potentially work with a more abstract or mathematical abstraction of a list. Best is ‘add this movie to your watch list’

Both are fine.

The correct sentence is: 2. Make sure to put this movie ON your watch list. Here's why: "On" is used when something is added to a list, schedule, or agenda. The idea is that you're adding the movie to a list that exists as a metaphorical surface, like writing a name "on" a piece of paper. In this case, you're putting the movie on your watch list. "In" is generally used when something is placed inside a container or enclosed space, which doesn't fit the concept of a list. So, we use "on" for lists, agendas, or schedules.

In

Answer:- 2. "Make sure to put this movie on your watch list. Reason we usually use On for something specific, when we want to do something. 'ON' is used to put it ON something, And 'IN' is used to put it IN something