The Marked Positions feature, available for Performance Tier subscribers, is enabled on the Maps page under Map Settings (gear icon). With Marked Positions turned on, a marker icon (an inverted teardrop shape) will appear on the left side of the Maps view.
Marked Position turned on in the Map Settings menu
Tap on the marker icon to add a marker during flight. A popover will appear with time and location information. Add an optional name or description to the marked position. Tap Save when done editing.
Marked Position description menu
Marked Positions persist and live with the Track Log if the Track Log is recording when the position is marked or are part of a flight plan created for the date and time of the flight on the Flights page. Please see this video for more details on how to use Marked Position: