You need an autoload script. Basically you can just create a node with a script attached and add a variable to hold a string. Then go into project > project settings. In the tabs you will see autoload. Go to the autoload screen and you can add any node to this, what this will do is automatically load the scene you add whenever you load any other scene, so it is the equivalent of adding it to each scene you load in the game.
What you can then do is change the variable in the autoload script to hold the text you want and then on your victory screen just look at the text in the autoload script and use that to assign to the label.