I wonder if it's already ok to send contents to Napkin and attach some information(URL, tags, etc.) at the same time. It seems not yet via iOS shortcut. It'll be really cool if there is an API(or URL Scheme?) for this. Hopefully, it can enhance Napkin's ability to quick capture fleeting thoughts and to integrate with other apps to a large extent.
The most ideal situation for quick capture, in my opinion, is, when you send something to Napkin, its URL is automatically attached so you can trace back to the source at any time.
And with an appropriate API, I can contact developers of some other apps, for example, MarginNote and Obsidian, etc.(Actually, I'm also a member of the early testers of the next generation of MarginNote, and I know several plugin developers of Obsidian), and ask them to develop plugins for integration with Napkin. It'll be so amazing if you can send your notes in MarginNote/Obsidian and attach their links to Napkin in bulk.