James - as a first step, try using webread to read the data from the URL which you can then pass to jsondecode like
urlToFetch = 'http://fantasy.espn.com/apis/v3/games/flb/seasons/2019/segments/0/leagues/15243217?view=mMatchupScore&view=mRoster&view=mScoreboard&view=mSettings&view=mTopPerformers&view=mTeam&view=modular&view=mNav';
jsonData = webread(urlToFetch);
As for logging into the website, that may be a little more difficult. Do you know of any public APIs that would allow this? I suspect that when you login, a token of some kind would be returned that you would need to use on subsequent calls to get the other data.