В стандарной теме prestashop это шаблон
/themes/default-bootstrap/modules/blocksearch/blocksearch-top.tpl
<input class="search_query form-control" type="text" id="search_query_top" name="search_query" placeholder="{l s='Search' mod='blocksearch'}" value="{$search_query|escape:'htmlall':'UTF-8'|stripslashes}" />