I imagine you'd have to get the private email from your iPhone, similar to how 1Password works.

And it appears from the image that the private email would be a bear to type into another device.

Hopefully, more devices will move away from requiring you to type anything into the device to sign in.

Quite a few devices or apps on devices have already done so. Instead of having you enter the account ID and password directly, they give you a URL to visit that is something like https://<device-maker>.com/add-device, and show you a random code.

You go to the given URL in your normal browser on whatever device you normally use for web browsing, where you can login to your account at <device-maker> and enter the code the device gave you.

A few seconds later the device notices you've completed this, and you are then automatically signed in on that device.