Ultimately, it comes down to your ability to convey the simple notion that "this is me" to someone who simply wants to get you out of his/her hair without being fired, but DESILVA might be a little harder to screw up than DE SILVA (assuming DE is part of your surname).

Remember, Chinese people typically have single character surnames (there are a handful of two character sets like 司马 that are so uncommon that many programmers don't account for them) and one or two character given names. As such, surname fields often don't allow for any spaces at all when they are occupied by Latin letters. Given name fields usually do, but not always. And, having a dedicated field for MIDDLE is quite rare.