Jumi application - calling a second php-file

[htmanager]

Hi, executing a php file with Jumi application works fine. But if I call another php file with <form action = ..>, there will be opened a site outside Joomla! and Jumi comes up with the error "Direct access is not allowed."
How can I execute the next php file(s) within Jumi (and Joomla!)?

Thanks in advance, Juergen