<template>
    <email>
        <custom_email_template translate="label" module="modulename">
            <label>Custom Email Template</label>
            <file>vendor/modulename/custom_email_template.html</file>
            <type>html</type>
        </custom_email_template>
    </email>
</template>

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Email:etc/email_templates.xsd">
    <template id="custom_email_template" label="Custom Email Template" file="custom_email_template.html" type="html" module="Vendor_Module" area="frontend"/>
</config>