public function section_callback() {
        echo '<p>Select which areas of content you wish to display.</p>';
    }

    public function field_callback() {
        $html = '<input type="text" id="input_offer" name="questionaire[input_field]" />';
        echo $html;
    }

    public function register_settings() {
        // First, we register a section. This is necessary since all future options must belong to a
        add_settings_section(
            'my_settings_section',         // ID used to identify this section and with which to register options
            'iPhones',                  // Title to be displayed on the administration page
            'section_callback', // Callback used to render the description of the section
            $this->plugin_name                           // Page on which to add this section of options
        );

        // Next, we'll introduce the fields for toggling the visibility of content elements.
        add_settings_field(
            'input_field',                      // ID used to identify the field throughout the theme
            'Offer',                           // The label to the left of the option interface element
            'field_callback',   // The name of the function responsible for rendering the option interface
            $this->plugin_name,                          // The page on which this option will be displayed
            'my_settings_section'         // The name of the section to which this field belongs
        );

        register_setting( $this->plugin_name, 'input_field' );
    }

private function define_admin_hooks() {

        $plugin_admin = new Questionaire_Admin( $this->get_plugin_name(), $this->get_version() );

    ...

        $this->loader->add_action( 'admin_init', $plugin_admin, 'register_settings' );

    ...

    }