Adding Tip/Donation To Existing Checkout Via GraphQL API

[NitinRana]

I'm working on a project that uses Shopify apis and I am able to create a checkout via graphql API update it, add promo codes finish the payment  and I'm doing all this through storefront graphQl mutations and I can't seem to find a way to add a Tip/Donation to the current checkout than finish the payment. 

I was looking for something like this

mutation {
    addTipToCheckout(checkoutID:"string",amount:"number"){
       checkout {
              id
         }
          userErrors {
              field
              message
         }
}
}