General developer forum

There are many ways to deal with this issue but for a start why don't you look into autocomplete field

https://docs.moodle.org/dev/lib/formslib.php_Form_Definition#autocomplete

With regards to the change of form this might help:

class myform extends moodleform {

    protected function definition() {
        $mform = $this->_form;

        $mform->addElement('select', 'myselect', 'select', [1 => 'first', 2 => 'second', 3 => 'third']);
        $mform->setType('myselect', PARAM_INT);

        $this->add_action_buttons();
    }

    public function definition_after_data() {
        $mform = $this->_form;
        if ($this->is_submitted()) {
            $data = $this->get_data();
            if ($data and !empty($data->myselect)) {
                $mform->addElement('header', 'aaa', 'bbbb');
            }
        }
    }

}

We check if the value of select is something other than 0 and if it is a random element is added.