Google Inbox doesn't appear to offer to handle
mailto: links as Gmail does.
Is there a way to enable this in Google Inbox?
Go to https://inbox.google.com and click the diamond-shaped icon to the right of Chrome's address bar (omnibox). Then choose Allow and Finished:
If you're using FireFox, Google doesn't present the handler-registration interface. However, all is not lost!
Open the developer tools with ⌥⌘i, and click the Console tab:
navigator.registerProtocolHandler("mailto" , "https://inbox.google.com/?mailto=%s" , "Inbox")
You should see a banner appear at the top of your Inbox window, asking if you want to allow Inbox to become a registered handler for
mailto: links. Click ‘allow.’
Next, you'll have to select Inbox as the desired handler inside FireFox itself,
… and finally, figure out a way to convince macOS to then select FireFox as the system-wide handler for
mailto: links. I keep the venerable-but-still-functioning (at least, as of the 10.14 “Mojave” beta, anyway) RCDefaultApp PrefPane installed for exactly this purpose:
Congratulations! You can test it out in the terminal, with a command like
open 'mailto:firstname.lastname@example.org'. Go forth, and verily shalt thou
You can try and set gmail to redirect to inbox in the inbox's definitions panel /Other