@JT_Morin on iOS device you can tap on the “tree” icon and select matches you want to add into your league.
Though I don’t think iOS “league” support is as elaborate as Android app has. As a simple workaround you could get a cheap Android tablet (Amazon Fire 7 is under $50 and often gets even cheaper on sales like Prime day, Black Friday, etc ) and then run Android app on it to get your “league” scores sorted out.
You can also run Android app in a free Bluestacks player on a PC or a Mac.