5
This is part of the XoopsFormClass, all the code for these can be found in the directory class/xoopsform. which will give you a good idea how each form works.
The form that interests you is formhtmltextarea.php
And this can be used in your form by using the following code:
include_once XOOPS_ROOT_PATH . '/class/xoopsformloader.php';
$sform = new XoopsThemeForm( _MD_SUBMITCATHEAD, "storyform", xoops_getenv( 'PHP_SELF' ) );
$sform->addElement( new XoopsFormDhtmlTextArea( _MD_DESCRIPTION, 'description', $description, 15, 60 ), true );
$sform->display();