There are many web sites which allow you to download a GPS trail in a standard format, such as a GPX file. This file contains all of the information MySports requires to synchronise trails with your watch.
For Running & Cycling trails you can both discover and create interesting trails on the following websites:
For Hiking trails, it is common for local or national tourist websites to curate the best walking trails for you to explore. A short internet search should let you discover these.
Once you have found an interesting trail you would like to use on your watch, export the trail as a GPX file and upload this file to MySports.