Since the input type is hidden, I'm assuming the user has no control over its contents. And as such, you might just as well save that data before you send it to the browser as doing it the other way around.
On the other hand, if the value of that hidden input is set by some user interaction through javascript, then obviously you will need to send the data to the server. If not by a form post, then you will need to do it some other way and ajax would usually seem the most reasonable. Moreover, if this is how it's supposed to work, you most likely won't even need that hidden input to begin with.
