For me, I stated how interesting I found their project, explaining my background and why I thought I was a good fit. I then said that I'd like to apply for the post and any more info on their plans for the project would be good to have.
Always seem eager and that their project is your main focus (even when its not)
If you get a nice reply, showing interest in having you for the post, you can then as if you could come in for a visit and face to face talk (informal) to get a clear indication of if its right for you; the supervisor, the project and the university.
Although in the end for me this didn't work out (my MSc supervisor offered me a phd) I did get several interview offers.
Good luck!