Arcpublishing is handled by the media route, which reads the site's own rendition list before choosing anything. No credentials are involved anywhere in this path — neither yours nor ours.
Either paste a Arcpublishing link above, or put tikt.com/ in front of any Arcpublishing URL in your address bar:
tikt.com/https://www.example.com/path/to/media
The order of operations
1. Copy the Arcpublishing link
Use the address bar or the platform's own share button — both give a URL the extractor recognises. Point it at the single item you want, not the page around it.
2. Let it read the renditions
Paste the URL above. The extractor asks Arcpublishing which renditions exist for that item and picks the best progressive one rather than assuming a quality.
3. Save what comes back
The file arrives in the container Arcpublishing served it in, and the extractor does not rebuild it — so what you end up with is what the uploader encoded, for better or worse.