You don't have to do anything. If you don't submit your turn on time, the AI will automatically take it for you.
Unfortunately, the AI is a brain-damaged sub-grade moron. What I really wish we had was an option to have our ships/empire just do NOTHING at all. This would almost always be better than having the IDIOT AI take our turn for us.
On the subject of PBW, I just can't say enough good things. I'm playing in Game 1 of the first round of Geo's Survivor Tourney. We are now in either the Last stages of the mid-game or the beginnings of the end-game. So far, this has been the best, most exciting game of SE4 I have ever played. I just had this HUGE, brawling, no-holds-barred fleet engagement against Golf Prez, and what a blood-bath that was.
The Phong had larger, more-modern ships, with more-modern weapons on them. But my brave Monkey People went up against his BC's and CA's in DD's and CL's, and we won
How did this happen, you ask? Aaaah, but that would be telling, wouldn't it?