I dont have any answers about HOW this was done, but those comments on unseenscreen are fake, as the actual video was just a promotional device.

I personally havent been on unseenscreen, but there are quite a few videos on youtube

I'm not completely convinced that it is to do with just filming her writing every letter off the alphabet, because the continuity is pretty flawless and it would be really slow if the website had to process your name and the letters and where to put them, plus, how would that explain her screaming whatever name you typed in at the end?