This is an accepted solution.
You cannot edit the checkout without a ShopifyPlus enterprise plan.
Try collecting the info during the cart step using a cart note
Or using cart attributes or line item properties for more granular information.
For advanced user If you are in a country whose delivery services frequently use landmarks then you might also want to change your checkout labels translations to try and get this info.
Be careful to avoid to confusing customers into entering undeliverable address information
Swap out THEMEID with the target themes id