Is it permissible for a marketer to appoint another marketer to market a product with an added commission without the owner's permission, if the platform allows it?

If the merchant does not stipulate that the marketer must do the work himself, then there is no harm in the marketer delegating another person to do the marketing, whether for compensation or without compensation. It also does not matter whether the compensation is less or more than what the marketer receives from the merchant, because the work required in marketing does not differ based on the individuals involved; rather, it is achieved simply by selling and realizing profit. Furthermore, a ju'alah (stipulated reward for a task) is valid even with an unspecified person.