Tiananmen square reservation to enter Forbidden City

Yes if you want to enter through the square. And there will be a long long line at the entrance of Wumen.

There is another solution: you can buy a ticket for Beijing Working People's Palace at a cost of CNY 2, walk through it to find the Wumen entrance to forbidden city.

What if I don't want to enter through the square?

Then you don't need the reservation for the square.