Load your project in IntelliJ, since it indexes your entire project, making it easier to search everywhere.
Then, you can either go to EDIT->FIND->FIND IN FILES, or press Ctrl+Shift+F and type in AutoLearnSkills.
In my case, there are 3 hits - 2 in player.properties (which is a txt file in the configs) and 1 in Config.java.
Now, your code has a config that is not present on my source, namely Auto_learn_3rd_skills. This means, that you have to create this manually in the Config.java and also take it out in the player.properties near the line with AutoLearnSkills = True/False. I won't be explaining how to do that for obvious reasons.
Then, you go back to IntelliJ and do a second search for Auto_Learn_Skills. You will have several hits, but you open the first in the Config.java.
There, you can CTRL+CLICK on the AUTO_LEARN_SKILLS, and it will show you all the functions that use it, like so:
As we can notice, two of these are in Player.java. Here, we have two options:
1 - add to these two function our check for whether the 3rd class config is set to TRUE or FALSE and whatever else you might need to add as a check.
2 - (the first one is superior/better in my opinion) Both of the uses in the Player.java use a function called RewardSkills(). As such, we can just add your code within that particular function and be done with it.