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The data format must be ISO 3166-1.
The country code with two uppercase letters.
For example, for Brazil it should be BR.

https://api.themoviedb.org/3/watch/providers/movie?watch_region=BR
or
https://api.themoviedb.org/3/watch/providers/movie?api_key=THE_KEY&watch_region=BR

@ticao2 said:

The data format must be ISO 3166-1.
The country code with two uppercase letters.
For example, for Brazil it should be BR.

https://api.themoviedb.org/3/watch/providers/movie?watch_region=BR
or
https://api.themoviedb.org/3/watch/providers/movie?api_key=THE_KEY&watch_region=BR

Yes, I use ISO 3166-1. But when I try to apply MY. It shows nothing. But previously, there is the result.

Today have result for MY (Malaysia) already. I will close the discussion. Thanks @ticao2 @superboy97