How to add specific phrase to end of search url

Question

This can be done in multiple ways But the easiest will be the following code.

The key is the GET method that will allow us to inject what you want.

You can try:

<div class="search-box">
    <div class="container">
        <div class="columns">
            <form method="get" class="nt-search-form" action="<?php echo home_url(); ?>">
                <input type="text" id="search-text" class="input-text" name="s" placeholder="<?php _e('Search &#8230;', 'customsearch');?>" autocomplete="off" />
                <input type="hidden" name="post_type" value="ait-item" />
                <input type="hidden" name="a" value="true" />
                <i class="fa fa-times close-button" aria-hidden="true"></i>
            </form>
        </div>
    </div>
</div>

Update

I marked out the =&a=true the first equal as this will be set the GET variable post_type to include the equal symbol.

If this actually what you want you to have to replace with this line <input type="hidden" name="post_type" value="ait-item=" />