The FORM tag has a TARGET attribute you can use to do this.

However, I should also note that you should not use FormMail. It has many security bugs in it--you can find details on virtually any other thread where someone mentioned the Accursed Name. (Quick runthrough: it doesn't use CGI.pm and doesn't have well-written CGI parameter algorithms; it doesn't use -T or any similar measures and AFAIK doesn't really check the validity of data; it calls sendmail without making sure the data it feeds it doesn't have evil things in it--basically, it acts as though Perl 4 is in use instead of Perl 5.)