If you look to the dark black box in the middle of the top of the screen you can see greyed letters: "Search games, ideas, posts". I have to admit, it's not really good visible and should be improved. If you click this box, don't expect to immediately write what you look for.

A new box appears and you have to click on text: "Are you looking for something?"

Now you can type in the key words you are looking for. If you want to search in Endless Space 2 forum only (i.e.) you have to deactivate the marks (to the left) relating to the other Amplitudes games (like Endless Dungeon, Endless Space, Endless Legend). Hope it helps.