DramaCategoryFragment.kt

class DramaCategoryFragment : Fragment(R.layout.fragment_drama_category),
    MovieListAdapter.FavouriteMovieListener {

    // ...

    private fun getDramaMovies() {
        val movieAPI = MoviesData.defaultInstance()

        // ...


        val retrofitData =
            movieAPI.getMoviesByCategory("39fd0a08c0cc7fd3041fc14605c22358", 18)
        
        // ...
    }
    
    // ...
}

MoviesData.kt

interface MoviesData {
    @GET("movie/now_playing")
    fun getData(@Query("api_key") apiKey: String): Call<MovieResponse>

    @GET("search/movie")
    fun searchMovies(@Query("api_key") apiKey: String, @Query("query") movieName : String): Call<MovieResponse>

    @GET("movie/{movie_id}")
    fun getMovieDetails(@Path("movie_id") movieId : Int, @Query("api_key") apiKey: String) : Call<MovieDetails>

    @GET("discover/movie")
    fun getMoviesByCategory(@Query("api_key") apiKey : String, @Query("with_genres") genreId : Int) : Call<MovieResponse>
  
    companion object {
        private const val BASE_URL = "https://api.themoviedb.org/3/"
      
        // Create a default instance here, make sure it's public. 
        fun defaultInstance(): MovieData {
            return  = Retrofit.Builder()
                .addConverterFactory(GsonConverterFactory.create())
                .baseUrl(BASE_URL)
                .build()
                .create(MoviesData::class.java)
        }
    }
}