Call this when you want to start the process of purchasing a product with a specific plan. Subscriptions can have multiple purchase plans which you can enumerate with GetInAppPurchaseSubNumPlans. To start a purchase with a specific plan get the plan token with GetInAppPurchaseSubPlanToken and then pass it to this command. Currently this command is only supported on iOS and Android.
InAppPurchaseActivateWithPlan( iID, planToken )