Ah, ok, then sorry, I guess that I misunderstood part of the question (I wonder if you all three are asking the same).
I have never used the pre-fill option, but certainly you don't need it for most usages to perform the task requested by the previous to users in this thread (if I understood correctly, of course)
You can use the tracker plugin to send data from a form displayed in a wiki page to fill the corresponding fields in a tracker. And you can choos whcich filed sto be shown in the form at the wiki page (tracker plugin), which fields you want to show in the tracker itself (or through plugins in wiki pages), and to whom show some fields, etc.
miyako, besides asking, can you help answering the other users also, please?
(otherwise this is not sustainable). I hope you understand...