etc/events.xml
<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
<event name="sales_model_service_quote_submit_before">
<observer name="staempfli_checkout_newsletter_subscription" instance="Staempfli\CheckoutNewsletterSubscription\Observer\NewsletterSubscription" />
</event>
</config>