How do you determine the Custom Field ID to add it as a tag in an email template?

Edit the custom field and the display ID is the one you need

We use many custom fields for various emails for different situations and it really is a bad way of doing it.  We use them so clients can be presented with specific fields based on the Request Type chosen when submitting a ticket in the web portal.  The problems start if you want to go back and change the order of the fields or add an additional one.  Since they are tied to Display Order ID then it changes order of everything after it and if you have emails being generated with action rules then all of the fields will be off and the emails will show the wrong ones.  I agree that using a custom field name/label is a much better way.  Another idea would be an option to just send the whole ticket with an action rule the way it would be sent if you CC'd someone.  There are a few "no brainer" features missing in WHD and this is one of them.

It would be even better if the Approval Process allowed you to use custom fields in things like an email to an approver. A good use case for that is a request that requires the user to enter the email address of a manager or supervisor. It would be nice to be able to set up an Approval Process to send an email to whatever that value is. Currently, you have to hard code a Client, CAB, or Location approver.