If you have a scale that supports ANT+ connectivity to your Garmin device, your weight data can be pulled in with your ride data whenever you sync your device.
Just go to Upload, and select your Garmin device. Any weight data will appear and be ready to upload.
Once uploaded, your weight data will appear on your calendar on your profile page as well as the Ride Center.
Don’t have an ANT+ Weight Scale? You can enter your weight manually by clicking on the day in the calendar view.
Want info more on ANT+ scales and how they work? Check out this article.