Here is a example how to make a new placeholder for Email templates and process that placeholder for a email.
So here we are adding placeholder for Ticket Type.
3a026ac8-e996-4a6c-9dc0-0883fa1b5ad2-image.png
Now you will see a placeholder in Email template with name Ticket type.
2. Process placeholder:
a943f6e1-173a-4ef5-8e8e-bd8f4a6b8b79-image.png
Finally clear your project cache by below command:
php bin/console c:c