Hello,
You can consider using the following code:
<form class="form-buscar row" method="get" action="artindex.php">
<div class="col">
<div class="form-label-group">
<input type="text" id="buscar" name="buscar" class="form-control form-control-lg" maxlength="25" required="">
<label for="buscar">Buscar por</label>
</div>
</div>
<div class="col-auto">
<button class="btn btn-lg bg-color text-white text-uppercase ls1 w-100" type="submit" style="padding-top:0.4rem;padding-bottom:0.4rem;"><i class="icon-search"></i></button>
</div>
<div class="text-center">
<small class="mt-5 text-muted fst-italic">A busca é feita nos títulos dos artigos</small>
</div>
</form>
This will definitely work fine. Hope this Helps!
Let us know if we can help you with anything else or if you find any further issues.