API Tracked Tournaments

Hello! So I want to make a certain format called a tracked format. In order to make this work I need to get a players historic stats from the fortnite api using
GET https://api.fortnitetracker.com/ we want to pull the total kills. (example)
and it comes back as 100 (example) Then I have the player join there game, start playing etc, And I pull there stats again and says you now have a total of 105 kills.(example) I subtract the ORIGINAL stat from the NEW STAT which gives me the score and then I make there score that. Please how can I do this?