Press "Add to Cart" on a product page and wait until the store has answered: the button goes "Adding…", "Added" and back, and the mini cart reloads. A refused add (a required option not chosen) ends the step as well: follow it with a message or a field error assertion.
Examples
When I add the product to the cart
And I add the product to the cart
When we add the product to the cart
When add the product to the cart
And we add the product to the cart
Pattern
When(/^(I |we )*add the product to the cart$/, …)
Part of @webship-js/magento-steps (github.com/webship/magento-steps), a step pack for Webship-js 2.0.x.