Back in the days, I created iOS Shortcut that uses Push-to-Kindle service to read web articles on my Kindle.
Link: Shortcuts
Is there a way to auto-fill email address in their form?
Glad I found Automators community!
Cheers!
Not an answer to your specific question but selecting the Kindle app from the share sheet works well for me.
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Atm, I choose above shortcut from the share sheet. It’s fast and I don’t need to fill, until I delete cookies from the browser 
What Kindle app do you mean? I found macOS version
On iOS I use Kindle’s own app from the share sheet and on the Mac I use the Push to Kindle browser extension.
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