If you want to have the field equal to the filename, simply set the field to
{filenamebase} or (if you want the filename extension) to
{filename}. If you want to apply this setting to multiple files, use the IPTC stationary pad, set the field to the mentioned value, and apply the stationary pad to the selected files.
This should do the trick!
Note: There are many many more "variables" like this that can help you accomplish all sorts of neat things
Cheers,
Hayo