in your view blade change to
{{ Form::open(['action' => 'postcontroller@edit', 'method' => 'PUT', 'class' = 'your class here']) }}
<div>
{{ Form::textarea('textareanamehere', 'default value here', ['placeholder' => 'your place holder here', 'class' => 'your class here']) }}
</div>
<div>
{{ Form::submit('Update', ['class' => 'btn class here'])}}
</div>
{{ Form::close() }}
actually you can use raw form like your question. but i dont recomended it. dan itulah salah satu alasan agan belajar framework, simple, dan cepat. so kenapa pake raw form kalo ada yang lebih mudah. hehe. proud to be indonesian.
reference: Laravel Blade Form