Just copy and paste this into your local [dokuwiki]/conf/userscript.js file.

/**
 * Add a placeholder text into the input field of the wiki search
 */
jQuery(function () {
 var defaulttext_searchinput = 'Enter Keywords to search wiki';
 
 var $searchinput = jQuery('#qsearch__in');
 if ($searchinput.length && !$searchinput.attr('placeholder')) {
 $searchinput.attr('placeholder', defaulttext_searchinput);
 }
});

Can be found in the javascript pages.