The following request packet turns on the FastCGI mode for the site with ID 7:
<packet version="1.6.3.0">
<site>
<set>
<filter>
<id>7</id>
</filter>
<values>
<hosting>
<vrt_hst>
<property>
<name>fastcgi</name>
<value>true</value>
</property>
</vrt_hst>
</hosting>
</values>
</set>
</site>
</packet>