If you are using default magento guest checkout flow then you must get it in "payment-information" api call. Please check below screenshot :

This api call fires when you click on "Place Order" button.

I hope this help to you.

Thanks,

If you are using default magento guest checkout flow then you must get customer email in "payment-information" api call request data. Please check below screenshot :

This api call fires when you click on "Place Order" button.

I hope this help to you.

Thanks,