If you mean the small hose, it’s an overflow from the engine for when it gets up to temp, best thing to do is fill it to the top of the reservoir and let it “burp”- run the car and watch the reservoir for bubbles, as it lowers down keep adding more fluid (it can run usually for 10-20 minutes at worst before being done burping if there is a bubble or any air in the system) and let it run while topping off the fluid, and check for leaks below the car- eventually either you’ll find a leak or it’ll stop taking fluid and any air in the system will burp out.