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.)
Are you posting in the right place? Check out Where do I post X? to know for sure.
Posts may use any of the Perl Monks Approved HTML tags. Currently these include the following:
<code> <a> <b> <big>
<blockquote> <br /> <dd>
<dl> <dt> <em> <font>
<h1> <h2> <h3> <h4>
<h5> <h6> <hr /> <i>
<li> <nbsp> <ol> <p>
<small> <strike> <strong>
<sub> <sup> <table>
<td> <th> <tr> <tt>
Snippets of code should be wrapped in
<code> tags not
<pre> tags. In fact, <pre>
tags should generally be avoided. If they must
be used, extreme care should be
taken to ensure that their contents do not
have long lines (<70 chars), in order to prevent
horizontal scrolling (and possible janitor
Want more info? How to link or
or How to display code and escape characters
are good places to start.