il form lo creato ecco il code

</ul>
<form class="form-inline my-2 my-lg-0" method="post" action="">
<input class="form-control mr-sm-2" type="search" placeholder="Search" aria-label="Search">
<button class="btn btn-outline-success my-2 my-sm-0" type="submit">Search</button>
</form>
</div>

per chiarire utilizzo bootstrap 4