gistfile1.js

// You can do that in your Chrome Console

// Load the initial object, like an Attempt

var attempt = new Parse.Object("Attempt");
attempt.id = "myId";
attempt.fetch();

// Get the relation "handler" from the attempt object
var relation = attempt.relation("products");

// Add some products to the relation. They don't need to be fetched, just to have an "id" property
var product = new Parse.Object("Products");
product.id = "id1";

relation.add(product);

// Let's add others product, we can use the same object as only the id matters
product.id = "id2";
relation.add(product);

// let's save the object to reflect the changes in Parse
attempt.save()