When I go into the System settings in Windows 10 and look at the "Default apps", the Email option provides Google Chrome, Mail, Microsoft Outlook, and "Look for an app in the Store" as my only options. I've tried right-clicking and shift-clicking. At the bottom of the list I've tried the "Choose default apps by file type", "Choose default apps by protocol", and "Set defaults by app". It seems that only a select set of programs are available as options for handling protocols (such as URLs and mailto links).
I've got a custom executable I'd like to use, but I'd be happy to find instructions for making any portable app the default app for a protocol in Windows 10.
Best Answer
I had a similar problem trying to get Chrome to be a default browser in a sandbox. After hunting around for about 4 days, I think I have it sussed and have the below .reg file which does the trick and makes Chrome show up in the default browsers popup in settings. you should be able to adapt for your needs.