Connect FitnessSyncer
You need a FitnessSyncer account. Open the developer page, create a personal application, then copy its client id and client secret into here. This browser remembers them.
That opens the approval in a new tab. When it is authorised and the page has loaded, copy the full address from that tab into here.
Connect Strava
Strava returns straight to this site. In the Strava API settings, set the Authorization Callback Domain to the host of {{ returnUrl }}.