Go to Newsletters of Newsletter at wordpress admin panel ( eg . https://wordpress.local/wp-admin/admin.php?page=newsletter_emails_index ).
Create the "new newsletter", and then choose any type of templates ( default presets ) except from Raw HTML. 
Input the simple test XSS payload at the Snippet Input
Payload : Test Snippet<script>alert(0)</script>

And Click the "Save" and "Next". The Stored XSS payload will execute.