onchange event select box in joomla

how can i put onchange in

$regionslistx[]= JHTML::_('select.option', "0", JText::_( '--No Location--' ),'value','text' );

or

$lists['regionslist' ]= JHTML::_('select.genericlist',  $regionslistx, 'City', '', 'value', 'text', $lx );

In the second case, where you have the empty argument, place the function you would like to use:

$lists['regionslist' ]= JHTML::_('select.genericlist',  $regionslistx, 'City', 'onchange=\"YOURFUNCTION()\"', 'value', 'text', $lx );

Also check the documentation: http://api.joomla.org/Joomla-Framework/HTML/JHTMLSelect.html