You can get
\bibpreamble to do this for you:
\bibsetup, as suggested in the comments, is only available if you use
biblatex. I was going on a hunch that you're using the
natbib package instead, which inserts
\bibpreamble just before the reference list, after the Reference/Bibliography heading.
If you were not using
natbib either, you may have had to use e.g.
etoolbox to patch the
thebibliography environment instead:
though the exact patch would depend on the
.cls you're using. I'm assuming the same