If you're utilizing Bootstrap (which I had not realized), my first suggestion would be to upgrade to Bootstrap 4.1.3. The library underwent a significant rewrite when transitioning from version 3 to 4, providing more input options in the newer release.
Regardless of whether you opt for version 3 or 4, you can implement something similar to the following:
<!-- SEARCHBAR-->
<div id="searchbar-wrapper">
<div class="row">
<form method="GET" action="index.html" ID="searchbar">
<div class="col-sm-6 col-md-6 mb-2">
<input type="text" name="destination" placeholder="Destination, city, address " />
</div>
<div class="col-sm-6 col-md-6">
<input type="submit" name="search" value="Search" />
</div>
</form>
</div>
</div>
For a Codepen example, check out this link
Additionally, here's a CSS snippet:
This text can go on with specifics about styles and design elements but is a unique revamp of the original content.