You could just save a small file in the user:// directory in Godot when the game ends, and read it as soon as the game loads.
Yes... I am not used to this thing yet, I tried to learn it but I am failing so far to do it without tutorial, anyway, that's not an excuse for me. I definitely need to learn it finally.
Hi, I have done it right now :)
And made an online leaderboard too, actually :D