Search Box Customization in WordPress Theme

Create a searchform.php file in your theme directory

(wp-content/themes/themename/searchform.php)

This File will replace your current search option in your theme.

searchform.php=>

<form method="get" id="search-form" action="<?php echo home_url(); ?>/">
<input type="text" value="Search" name="s" id="search-input" onfocus="if (this.value == 'Search') {this.value = '';}" onblur="if (this.value == '') {this.value = 'Search';}" >
<input type="submit" id="search-submit" value="" >