In order to write your own custom module in drupal 8/9, you need to create basically two files as below, if you would like to name your module to "dcodes_custom_rules_action":

dcodes_custom_rules_action.info.yml
dcodes_custom_rules_action.module

Here might be the code for file ".info.yml":

name: Dcodes Custom Rules Action
description: Custom Module By Dewan For Node Create Event
package: Dcodes
type: module
core: 8.x

Here might be the code for file ".module":

<?php