Georgeky is correct, as usual. But with his kind permission, I'd like to add a couple of things:
(1) Be careful removing that plug! Sometimes there is a little residual air pressure inside the reservior and you can get it in your eyes! Hydraulic fluid is toxic, so be careful!
(2) Where is your leak? The hydraulic fluid is also lubricant for the pump. If you run the tractor with the fluid low, you can damage the pump. Fix the leak if it is at all convenient.
(3) This one will be controversial. Use genuine IH Touch-Control fluid. I used something else once and the paint immediately deteriorated inside the reservior!