Kindly, Joakim
You can use an analysis board to enter the Gane and either use the Save option there or move it to a Library collection.
https://support.chess.com/article/693-how-do-i-use-the-analysis-board
https://support.chess.com/article/1987-how-do-i-use-the-chess-com-library
Kindly, Joakim