Déclaration de l'observer

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="customer_register_success">
+        <observer name="myCustomerRegisterSuccessEvent" instance="NicolasBejean\Customer\Observer\CustomerRegisterSuccess" />
+    </event>
+</config>