Start on the Templates tab, and click the ‘Add a new template’ icon:
Choose "Import from URL", then paste in the URL of your email template and click "Create in Classic Builder":
Please note:
Unless importing from an existing CRM template, your import URL must end in
.htmlor the import won't workUsing third-party code can require a little tidying up before it is 100% ready. If you get any warning messages upon saving, see ‘Troubleshooting’.