Checkout provides URL parameter to directly add & pre-select a products to the cart using parameter.
Take a look at this parameter pattern -
https://www.yourdomainhere.com/checkouts/YourAeroCheckoutPageName/?aero-default=Product_Sequence_Number
Let's say if we have four products in Product list on Checkout page and we want to add third product to the cart, then we will use parameter as:
https://www.yourdomainhere.com/checkouts/YourAeroCheckoutPageName/?aero-default=3
You can also add multiple products to cart (when checkbox option selected), by comma saperated. ie. /aero-default=1,3
Final Ouput:
