In order to add to cart, you must have an eCart object.
the eCart object defines the cart columns and other cart settings like tax, shipping, or discounts.
the add to cart behavior adds item information defined by the eCart object.
you cant bypass creating the ecart object. It needs to exist in order to use the add to cart behavior to add items to the cart.
to add an item to the cart by landing on a page, use the add to cart server behavior.
open the server behaviors panel, and click on the plus button, select eCart -> Add to cart -> add to cart and use the On Page load trigger.