To set default apps in portal.office.com, start by logging into the portal with your credentials. Once logged in, access the Settings gear icon in the upper right corner of the screen. Select "App Settings" or "Preferences," depending on your version. From there, navigate to Default Apps, where you can choose your preferred applications for email, calendar, and other services. Customizing these settings in portal.office.com helps ensure a seamless workflow, automatically opening selected apps for specific tasks. Make sure to save any changes you make to activate your default settings.