He's gorgeous and looks healthy! Lovely colour and a good size. They quite commonly get a sticky eye when they get older for some reason and I think Cookie is not a young hamster, is that right? Sometimes it just frees itself up again but it's not very nice for them if it lasts longer than overnight.
The easiest thing to do is just gently dab it with saline until it frees up, which it tends to do quite quickly. When I say dab it I don't mean get him soaking wet! So just the corner of a soft cloth (eg a microfibre cloth) or some people use a cotton bud and wring it out so it's only just a tiny bit wet - a bit more than damp. And gently dab it two or three times and it should start to free up.
To do this, if he's a bit wriggly, you might need to swaddle him in a blanket or something - ie wrap it round him and hold him upside down and the blanket will hold his arms and legs in place (maybe not for long!) so he can't wriggle free.
I've had to do it a few times and no they don't like it, but once the eye is freed up they seem so pleased about it that they forgive you straight away!
To make saline you just put a teaspoon of salt in a pint of boiling water. But the boiling water is just to dissolve the salt and you then need to let it go cold - which can take a while. It doesn't have to be completely cold - skin temperature is ok, but definitely not hot or too warm.