It's in the guidelines as an image under "Specs":
You can also use the Material Component documentation for developers to see how it's implemented on other platforms and to and view a demo.
EDIT: Rereading your question, I have a better understanding of what you were going for. There isn't a specified standard for default buttons as separate from focused buttons, and I'd suggest you either:
a) use the "focused" state. For all intents and purposes, it's as if the button was focused, as it can be interacted with with a keyboard. Yes, two UI elements might be focused at a time—that isn't a problem, as both can be interacted with with the keyboard. Keep in mind that, if your form contains e.g. checkboxes or radio buttons, a default button could get in the way—sometimes enter would submit, sometimes it wouldn't...
b) don't differentiate a default button—use standard button appearance. "Enter" is generally used for submitting something, whether it be in a search field or an address bar or in a simple form. It's a common pattern that's relatively well-known. If you're not using "Enter" for submission, then I ask you to reconsider, as tying e.g. "Cancel" to the enter key would be grossly counter-intuitive.
Either way, neither of these might be immediately understood by users (you can test it and see). If you want a clear indication of what "Enter" will do, you can always add a text label like "Press enter to submit".