If you're generating images from HTML on the fly, you might not want to save the HTML anywhere. Hosting generated pages in a place that Urlbox can reach can also be challenging.
What if you didn't need a URL for the things you want to turn into images?
You can post HTML, CSS and even JavaScript to Urlbox. URLs not required!
Here's an example:
curl -X POST https://api.urlbox.io/v1/render/sync \
-H 'Authorization: Bearer YOUR_URLBOX_API_SECRET' \
-H 'Content-Type: application/json' \
-d '{
"html": "<div><h1 id=\"title\"></h1><p>Testing</p></div>",
"js": "document.getElementById(\"title\").innerText=\"Hello World\"",
"css": "div { padding: 10px; background-color: red } h1 { text-align: center; color: white } p {text-align: center; color: white}",
"selector": "div",
"width": "400",
"height": "300"
}'
It results in the following image:

You still have the full power of Urlbox's rendering engine so you and your designers can make far more beautiful images this way complete with fonts, emoji and anything else you'd expect to render nicely in a browser.