You can add a Favourite in the Social tab. Search for a player by their username and user tag. Once the player has been found, click the add favourite button.
To add a player to your Favorites list after the match, please tap on their profile icon on the results screen. This will pull up their player profile. Then, tap the "+" icon in the upper right-hand corner of their profile. Once a request has been sent, they will appear in your favourites list.
You can remove a Favourite by visiting their profile and tapping the "Remove" button under their username.