To complete Pit Pass milestones you need to collect Trophies. Trophies come from two sources - playing Duels and participating in Events.
In Duels, Trophies are only awarded when racing in your highest and second highest unlocked Series.
When racing in your highest unlocked Series, three Trophies are awarded for a win and one Trophy is awarded for a loss. One extra Trophy is awarded for having the fastest lap time, regardless of the race’s outcome.
When racing in your second highest unlocked Series, one Trophy is awarded for a win and no Trophies are awarded for a loss, or the fastest lap.
No trophies can be lost during Duels or Events.
Trophies are awarded for every race in Events, regardless of the selected difficulty tier. The number of awarded Trophies is the same as in your highest unlocked Series for Duels.
No Trophies are awarded for forfeiting a Race.
The number of Trophies required to complete milestones increases as you progress in the Pit Pass. Rewards can be claimed after the end of a Pit Pass, however, Trophies cannot be collected anymore once a Pit Pass expires.