How to use Instant Purchase in Magento2.2.2

You can find answers in the official documentation:

• http://devdocs.magento.com/guides/v2.2/mrg/ce/instant-purchase/
• http://docs.magento.com/m2/ce/user_guide/sales/checkout-instant-purchase.html

Example of payment method integration may be found in Braintree module:

• https://github.com/magento/magento2/tree/2.2.2/app/code/Magento/Braintree/Model/InstantPurchase
• https://github.com/magento/magento2/blob/2.2.2/app/code/Magento/Braintree/etc/config.xml#L74-L78