do I always get the -1 / +2 attack

or only when I declare that Im going to use it ?

and is it optioanl or always in effect when I make a close combat attack ?

First, all feats are considered optional with the exception of a rare few. Haymaker is not one of those few.

Second, Haymaker requires that you give the character a close combat action, not attack. So, even if it wasn't optional, you could always do something like CCE or HSS that would not allow you combo the Haymaker... luckily, it is optional and we don't have to jump through those hoops (but we could, if we had to).

Just as a side note, to think about it logically, if you could always get the +2/-1, why would anyone ever take the push?