curl --request POST \
  --url https://api.example.com/api/v1/templates/preview \
  --header 'Content-Type: application/json' \
  --data '
{
  "content": "<string>",
  "template_id": "3c90c3cc-0d44-4b50-8888-8dd25736052a",
  "custom_variables": {},
  "contact_id": "3c90c3cc-0d44-4b50-8888-8dd25736052a"
}
'

{
  "preview": "<string>",
  "all_variables": [
    "<string>"
  ],
  "builtin_variables": [
    "<string>"
  ],
  "custom_variables": [
    "<string>"
  ],
  "missing_variables": [
    "<string>"
  ]
}