How do I fire a checkoutStarted event with the cart on my Hydrogen storefront?

Auto tracking already gives me product views and cart views, which is great, but I want an explicit checkout-start signal that carries the cart contents so I can use it as an experiment goal later.

How do I send it, and what does teh payload look like? I have a checkout button handler where I can drop the call:

function onCheckout() {  // fire event here}