1. create a env file in <oracle_home>\reports\server\, suppose it's test.env
    append the directory where the fmx is in to the parameter FORMS90_PATH=
    and do other necessary modifications.

2. modify web.xml in <oracle_home>\j2ee\OC4J_BI_Forms\applications\forms90app\forms90web\WEB-INF
add the envFile parameter into the servlet's definition
   <servlet>
      <servlet-name>l90servlet</servlet-name>
      <servlet-class>oracle.forms.servlet.ListenerServlet</servlet-class>
      <init-param>
         <param-name>envFile</param-name>
         <param-value>C:\Oracle\ias\forms90\server\geo.env</param-value>
      </init-param>
   </servlet> 

3. modify the initial html file generated by form servlet
change
<PARAM NAME="serverURL" VALUE="/forms90/l90servlet?ifcfs=/forms90/f90servlet">
to
<PARAM NAME="serverURL" VALUE="/forms90/l90servlet">