From the Dashboard select, "Club management", followed by "Roster Builder", here you will select the event from the drop-down and hit "Submit".
Here you will drag and drop the player to the roster. (Skip to step 4 if he is already on the roster)
Once you drag and drop the player a window will appear where you will select the Position, Jersey (optional), and Status. For a Secondary Player, you will mark them as S.
You will now see the player on the right-hand side in the roster with status set to "S". *If you need to change the status of the player, here you can change it from the drop-down list.