Sequelize how to use association table?

I'm having an issue with Sequelize because I don't know how to approach the problem.

I have 3 tables : A (game), B(platform) and AB (game_platform). A can have 1 to many B and B can have 0 to many A. To do that I made an association table : AB.

In Sequelize I created the A,B,AB models then I did :

db.Game.belongsToMany(db.Platform, {as: 'Game', through: 'GamePlatformsJoin', foreignKey: 'game_platforms_fk_game'});
db.Platform.belongsToMany(db.Game, {as: 'Platform', through: 'GamePlatformsJoin', foreignKey: 'game_platforms_fk_platform'});

Now this is how it will go : On my website platforms will be created and then games. When a game is created the user will need to define the platform(s) associated to it. But how can I tell Sequelize that I want a new entry in my association table ? I can add it as with any other table but is there a simpler way ?

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