It is a Custom Activity Field, basically you create it and give it the information you want (with or without javascript code). Here you can find more info about this feature:
Custom activity fields - Intervals.icu Forum
Computed activity fields - Announcements - Intervals.icu Forum
Once created, it will be available for any activity and you can fill it manually with a specific value. If you want, you can click on search and find it if this is easier for you.