2002 Chevy Malibu Adding Coolant

The cap on my car's coolant expansion tank went bad, allowing all the coolant to leak out. When I went to add coolant, I noticed that there is no access to the radiator directly to add coolant. Do I just fill up the expansion tank or is there some secret radiator opening that I can't see from the top?

I just want to make sure it's topped off. I am getting ready to go on an extended drive.

Thanks!