Where can I hide a link on the webpage

best option is to hide inside a DIV as rena said

 

I wouldn't try to hide a link at all. What you could do it make a small footer link to a separate links page which has your link on it. You won't get as much link equity as if you had your link on the main page but it's still better if your boss doesn't want clients to see it.

 

I would not even go there your boss is delusional as most bosses are. Unless you work for yourself of course. Then your paying customers are your boss. If they are delusional charge them extra.

What does this have to do with your question? Everything.

They want you to advertise their site but not make it visible? This makes no sense to me.