Races are created in the app, and you need an account with a confirmed e-mail address.
iPhone: tap the + next to the tab bar.
Android: tap the + at the top right of the Run screen.
Under Event type, pick one:
Custom race: give it a name and, optionally, a start time. Then choose I have a GPX and upload the course file (max 8 MB), or I don't have a GPX and enter the race distance in km.
Catalog: pick your race from the list. The name, date and route come from the catalog. Can't find it? Submit it, so others can use it too.
Free run: a training run without a route.
Then pick the Entry mode (how followers get in; you can change it later in Race settings), and tap Create event.
You get a Race ID (8 characters) and an invite link to share. In Race ID + access code mode you also get a 6-character access code.