GET:
to set the method your form is send in cakephp3 as GET;
echo $this->Form->create(null, ['type' => 'get']);
to get the Data sent via get in controller;
$this->request->query['txtboxfromform'];
POST:
//set the method echo $this->Form->create(null, ['type' => 'post']);
//get data $this->request->data['txtboxfromform'];
Forms in cakephp3:
http://book.cakephp.org/3.0/en/views/helpers/form.html