try ...
Code:
<input type="text" name="posted_data[substring]" {if $search_prefilled.substring|escape}value="{$search_prefilled.substring|escape}"{else}value="Enter search term" autocomplete="off" onKeyUp="ajax_showOptions(this,'getProduct',event)" onfocus="if(this.value=='Enter search term') this.value='';" onblur="if(this.value=='') this.value=this.defaultValue;"{/if} />