Data from a Garmin is the best way to capture your data as a stationary activity at the moment. This means that only file formats in TCX or FIT can be uploaded as a stationary activity. If you don't have any accessory data, like heart rate, you can upload GPX files and mark the activity as stationary as well.
It is best to turn OFF your GPS while recording a stationary activity. This will help with correct reporting of moving time and with distance recorded in your data (if you have a speed sensor or power meter). Also, for Premium members with Suffer Score analysis, your HR zone breakdown will be correct.
Didn't record an activity with your Garmin? Enter a manual activity. You'll get the credit for the training and all you have to do is enter the basic fields like duration, time of day, date, and distance. To create a manual activity go to "Upload Activity" on your account and choose "Upload an activity manually".
For more information on Stationary activities on Strava, go here.