An example (from WooCommerce Quick View Pro) of our current plugin license setup, including adding the license setting to the settings page. This is to be replaced by a new license system currently in development.

old-plugin-license-setup.php

<?php
final class Quick_View_Plugin {

	const NAME	 = 'WooCommerce Quick View Pro'; // should match download name in EDD
	const VERSION	 = '1.1';
	const FILE	 = __FILE__;

	private static $BASENAME = null;

	/* The plugin license manager. */
	private $license;

	/* The singleton instance. */
	private static $_instance = null;

	private function __construct() {
		$this->includes();

		// Create our plugin license & updater.
		$this->license = new \Barn2_Plugin_License( self::FILE, self::NAME, self::VERSION, 'wc_quick_view_pro' );
	}
	
	// rest of class...
}

// use Barn2\Lib\Util as Util;

class Admin_Settings_Page {
	
	private $license;

	public function __construct( \Barn2_Plugin_License $license ) {
		$this->license = $license;
	}
	
	public function register() {
		// Register extra setting types with WC.
		foreach ( array( 'hidden', 'settings_start', 'settings_end' ) as $field ) {
			add_action( "woocommerce_admin_field_{$field}", array( '\WC_Settings_Additional_Field_Types', "{$field}_field" ) );
		}

		// Add our settings & section.
		add_filter( 'woocommerce_get_sections_products', array( $this, 'add_section' ) );	
		// Add Quick View settings to Products tab.
		add_filter( 'woocommerce_get_settings_products', array( $this, 'add_settings' ), 10, 2 );
		
		// Sanitize and save license setting.
		add_filter( 'woocommerce_admin_settings_sanitize_option_' . $this->license->license_key_option, array( $this->license, 'save' ) );
	}
	
	public function add_section( $sections ) {
		$sections[self::SECTION] = __( 'Quick view', 'woocommerce-quick-view-pro' );
		return $sections;
	}
	
	public function add_settings( $settings, $current_section ) {
		// Check we're on the correct settings section
		if ( self::SECTION !== $current_section ) {
			return $settings;
		}

		$plugin_settings = array();

		// Add license key settings.
		$plugin_settings[]	 = array(
			'title' => __( 'Quick view', 'woocommerce-quick-view-pro' ),
			'type' => 'title',
			'id' => 'quick_view_license_section',
			'desc' => '<p>' . sprintf( __( 'The following options control the %sWooCommerce Quick View Pro%s extension.', 'woocommerce-quick-view-pro' ), '<strong>', '</strong>' ) . '<p>'
			. '<p>'
			. Util::barn2_link( 'kb-categories/woocommerce-quick-view-pro-kb/', __( 'View Documentation', 'woocommerce-quick-view-pro' ), true ) . ' | '
			. Util::barn2_link( 'our-wordpress-plugins/support-documentation/', __( 'Get Help', 'woocommerce-quick-view-pro' ), true )
			. '</p>'
		);
		$plugin_settings[]	 = array(
			'title' => __( 'License key', 'woocommerce-quick-view-pro' ),
			'type' => 'text',
			'id' => $this->license->license_key_option,
			'desc' => $this->license->get_license_key_admin_message(),
			'desc_tip' => __( 'The licence key is contained in your order confirmation email.', 'woocommerce-quick-view-pro' ),
			'class' => 'regular-text'
		);
		if ( filter_input( INPUT_GET, 'license_debug' ) ) {
			$plugin_settings[] = array(
				'type' => 'hidden',
				'id' => 'license_debug',
				'default' => '1'
			);
		}
		if ( $override = filter_input( INPUT_GET, 'license_override', FILTER_SANITIZE_STRING ) ) {
			$plugin_settings[] = array(
				'type' => 'hidden',
				'id' => 'license_override',
				'default' => $override
			);
		}
		$plugin_settings[] = array(
			'type' => 'sectionend',
			'id' => 'quick_view_license_section'
		);
		
		// Add more settings...
	}
	
	
}