How to make a search box move with the page?

Hey folks,

Have built this very basic site for a friend as web design pratice. Was wondering how I can get the search box to move with the page. So if you scroll down the page and can no longer see the menu bar, the search box will still be available. Any help appreciated.

https://autodiction.com/

In case it helps, here is the code in searchform.php

<form method="get" id="searchform" action="<?php bloginfo('url'); ?>/"><div class="search-box">
<input type="text" value="search this site" name="s" id="s" onfocus="if (this.value == 'search this site') {this.value = '';}" onblur="if (this.value == '') {this.value = 'search this site';}" />
<input type="submit" id="searchsubmit" value="Search" />
</div></form>