You can add players to a scout list so that you will be notified as soon as they are on the transfer market. To add a player to the scout list, go to 1st or 2nd Bundesliga in the app. On the top right you can search for the player's name with the magnifying glass and then on the top right with the three dots "Add to scout list". Alternatively, you can add players who are on the transfer market to your scout list.