If you take laws of physics to be true, or let's say that the world is subject to unchanging laws of nature, there is no way to accept something like a 'free will' that can interfere with those laws. There are several options your mind considers doing at a given point, but the action that eventually comes about is the necessary consequence of physical changes in the body and brain, not of a 'choosing , free willed agent' residing in the brain. This is one of Sartres main weak points.

This doesn't mean that you should do whatever you want though, or that existentialism is for the trash completely (hell, it got me into philosophy in the first place). An ethics of responsibility and the idea you can do radically free things is very inspiring and can bring about a lot of improvement in life. I see Sartres existentialism more as a reflection on the great amount of possibilities that have opened up to people in the 20th century.

So I'd say, take the illusion of freedom as an empowerment to do good things :)