How to automatically add a product to the shopping cart in OpenCart 1.5.6.* from another site and proceed to checkout?

Question

PHP

Hello dear experts!
The essence of the problem: there is a landing page on a regular html with the "Order" button, you need to do that by this button
1. the corresponding product in the store on OpenCart was added to the cart
2. the cart was automatically switched to the checkout mode (entering the delivery address, payment method, etc. .)
3. The user from the landing page was redirected directly to the cart with the order.
Those. hide the first two points from the user and do it automatically, and the user will immediately be shown the address entry form, etc.
Tell me, please, how to do it?