Yes, that looks right, but the 'Playtomic' variable can be renamed by the minifier too. It needs to be accessed by [""] syntax as well. The best thing to do is a global variable within the anonymous function for the plugin code, e.g.
var myPlaytomic = window["Playtomic"];
The external script has to have finished loading before this will work! This then always refers to the global variable by that name. Then, use
myPlaytomic["Leaderboards"]["Save"]
and so on. The syntax isn't very pretty, but it's really important to stick with it for the minifier to work.