The appendix is what is appended to something. What do we call something that is prepended to something else?
When describing parts of a book, the "preface" or "prologue" are common.
Accordingly to http://encyclopedia2.thefreedictionary.com/prepend there is no such thing as "to prepend"
The correct English word is "prefix;" for example, "the header is prefixed to the packet." See prefix.
prefix: The beginning or to add to the beginning