Unfortunately, Duo does not have a way to configure a default authentication method.
Rather than setting a default authentication method, Duo will always prompt you for your last used authentication method. If you select an option and authenticate with it, Duo will remember and continue to automatically prompt you with it until you authenticate with a different method. To choose a different authenticated method from what is provided initially in the prompt, follow these steps.
1. From the verification prompt, select Other options.
2. Choose the option you wish to use, and complete logging in. Your options will depend on your enrolled devices.
3. The next time you log in, the Duo verification prompt will ask you to use this same method.