request unique name using property expansion

Hi,
I am trying to insert unique user name into RESTful request using "NewTestUser+new Date().format('MMddyyyyHHmmssSSS'))" and can't get it to work. Here is an example of the newUser field value I want get inserted into request: NewTestUser11092010093945318

here is what I have been trying
"newUserd":"{=NewTestUser+new Date().format('MMddyyyyHHmmssSSS')}"
"newUserd":"{context.expand(NewTestUser+new Date().format('MMddyyyyHHmmssSSS'))}"

How do I write property expansion to create unique user name in my request?

thanks