is there a way to add a custom eula agreement to my game befor downloading or purchasing what ever works for me 

You can release your game with an installer - that way they have to accept your EULA or terms and conditions before installing the game.  However, it won't allow them to play the game through the itch.io console. 

Alternatively you can just put a disclaimer in your description that states what your terms are with the clause - by clicking download / or playing this game you agree that you have read these terms and will abide by them.