Using "https:" also works (at least with the Brave mobile browser on Android)

https://www.zotero.org/save?type=

URL for adding URL, ISBN, DOI, PMID, or arXiv IDs to one's Zotero account.

I've created a mobile shortcut using the URL Forwarder app to accomplish this with a share functionality after highlighting an ISBN.

Might also try using https://play.google.com/store/apps/details?id=com.srowen.bs.android&hl=en with the added custom search query custom search URL https://www.zotero.org/save?q=%s to see if that might work as well. This should allow using a scanner to get ISBN barcodes into the system as well. Useful for browsing at the bookstore.

I should also create a javascript bookmarklet for this pattern as well.

See also: - https://forums.zotero.org/discussion/77178/barcode-scanner - https://forums.zotero.org/discussion/76471/scanning-isbn-barcode-to-input-books-to-zotero-library

Alternate URL paths for this: - https://www.zotero.org/save?type=isbn - https://www.zotero.org/save?q=