API V1 PHP Start payment plan on new order

I’m developing an e-commerce custom checkout.
Keap API rest v1.
Lang PHP

During the checkout the user is able to choose 1 time or plan. I’m all right with the first option.
Instead, I do not know how to start a plan on a new order, when user does not want to pay all in one tranche.

I think I found the answer. If a product has a payment plan associated, it will auto-trigger it.
No need to trigger it via API