Add to cart a subscription variant via ajax in shopify

We need to add a recharge payments subscription variant (https://majuelos-singulares.myshopify.com/products/club-majuelos-singulares-nivel-1?variant=39940238442702) to cart when a button is clicked.

We have some tests here -> https://majuelos-singulares.myshopify.com/pages/crea-suscripcion-clasica

We are following their instructions at doc but it is not working
https://support.rechargepayments.com/hc/en-us/articles/360008683774-Add-an-Item-to-Cart-with-AJAX

So we just need on this page -> https://majuelos-singulares.myshopify.com/pages/crea-suscripcion-clasica -> when click one of the buttons the variant of the subscription to be added to the cart

Info Added:
- Code of the AJAX query we are using-> PAGE.TOCARTMS.LIQUID and screenshot -> PAGE.CODE.PNG
- Code at the page where to add the button -> https://majuelos-singulares.myshopify.com/pages/crea-suscripcion-clasica -> PAGE.CODE.HTML and screenshot -> PAGE.CODE.PNG
- Page of the recharge payment subscription in shopify -> https://majuelos-singulares.myshopify.com/products/club-majuelos-singulares-nivel-1

We need:
- On the page -> https://majuelos-singulares.myshopify.com/pages/crea-suscripcion-clasica with 1 button. When the button is clicked then > add to cart 1 of the recharge payment variants
- The location of the new code to be added for working. We also need the code documented to follow the logic of the code.