Checkout provides URL parameters to directly add & pre-select products to the cart using parameters.

Take a look at this parameter pattern:

https://www.yourdomainhere.com/checkouts/YourAeroCheckoutPageName/?aero-default=Product_Sequence_Number

Let's say, we have four products in the Product list on the Checkout Page and we want to add a third product to the cart, then we will use the parameters as:

https://www.yourdomainhere.com/checkouts/YourAeroCheckoutPageName/?aero-default=3

You can also add multiple products to the cart (when the checkbox option is selected), by separating with a comma, i.e. /aero-default=1,3

Final Output:

This is how you can select a particular product using the Checkout URL parameter.