How to associate selling plan to a checkout via Checkout Storefront API

[hoanginnomize]

I am building a custom app for our client, we have to create and complete checkout and redirect to the checkout page. We've completed that flow, but now we want to associate the right selling plan based on our condition (automatically). I have checked the checkout API but I cannot see any parameters that we could use to link a selling plan to the checkout object. 

Are there any references that we can use to implement this scenario? We are using PayWhirl for our recurring payment. 

[JaiSingh]

@hoanginnomize  Hi I'm searching for the same, I need to assign selling_plan to my checkout. Could you suggest if you find any solution?