1. Enable Create Account After Purchase Feature.

To make the Create Account After Purchase feature available on your checkout page for guest customers, you first need to enable and configure it in Admin -> Default Settings -> Customers or in Admin -> Stores -> Your Store -> Customers section.
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2. Create Account After Purchase Feature on the Checkout Page

Based on your configurations, Create Account After Purchase feature can appear on the Checkout Page in two different ways:
  1. If you've set "Allow Customer To Create An Account With Purchase" to "Yes", automatically checked / unchecked (based on "Checked by Default?" config) "Create an Account" checkbox will be displayed near the "Place Order" button.
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  2. If you've set also "Always Create An Account With Purchase" to "Yes", message "An account will be created automatically with your order" will be displayed near the "Place Order" button. 
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3. Customer Account Creation Process

If guest customer confirmed that he wants to create an account during the purchase or if your Store was configured to create an account automatically, customer account will be created after the checkout process and the newly created order will be assigned to the created account.

If the customer with the specified email address already exists within the system, the newly created order will just be assigned to the existing account.