Simply open an elevator and connect it to LTN with the switch on the top of the elevator window. As soon as you do the LTN networks on the ground and in orbit will be merged and deliveries will be scheduled between them. This uses the new LTN API introduced in version 1.18.0.

Using LTN through SE's elevators has a few constraints:

• Trains are always sourced from the same surface as the provider station. Effectively you need depots both on the ground and in orbit and trains will return to where their depot was.
• The setting "Delivery completes at requester" must be disabled because it clears a train's schedule as soon as the train leaves the requester. That would stop the train from returning to the original surface and thus create an imbalance in the train pools.
• The network-ID of an elevator will act as an additional mask when deciding which deliveries can go through it. If you don't use network-IDs, you can leave the elevator's ID at the default of -1 which simply allows all network-IDs to go through.
• Elevator stops don't receive temporary stops like logistic stops do. This means Factorio's train pathfinder is free to drive to the closest elevator with a matching name. If you intend to assign different LTN network-IDs to the elevators on a surface it's best to give those elevators unique names via the elevator UI.