Here is the easiest way I have found.
First, make sure your preferences has show internal ID. To do this, go Home>Set Preferences>under the General Tab on the right make sure "Show Internal IDs" is checked.
If this is done, then go to Documents>Templates>Email Templates and it will show the internal ID in the list.
If you recently made the Template, a quick-tip is to sort by internal ID descending in the list and the most recent Template will be shown. If it isn't a new template, I use just good ole' Ctrl+F to search my list of Templates if I know it's name.
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