In order to add custom permissions we used to use hook_permission in Drupal 7, however in Drupal 8 this is done through the yourmodulename.permissions.yml file
Here is an example of what the file should look like
administer commerce_amazon_slider: title: 'Administer Commerce Amazon Slider' description: 'View, edit and update Commerce Amazon Slider configurations'
After you add this file with the customn permissions you need, clear the cache then navigate to YourSiteName.com/admin/people/permissions to view the new permissions
Thank you and looking forward to help you do more with Drupal