Inventory_Presser_Addon::add_license_key_box( mixed $option_name, mixed $settings_section, mixed $sanitize_values_hook_name, mixed $license_key_key_name = 'license_key' )

		public static function add_license_key_box( $option_name, $settings_section, $sanitize_values_hook_name, $license_key_key_name = 'license_key' )
		{
			add_action(
				'admin_init', function () use ( $option_name, $settings_section, $license_key_key_name ) {

					//is $settings_section an existing section?
					global $wp_settings_sections;
					if ( empty( $wp_settings_sections[INVP::option_page()][$settings_section] ) ) {
						//no, abort
						return;
					}

					$option = get_option( $option_name );
					$current_value = empty( $option[$license_key_key_name] ) ? '' : $option[$license_key_key_name];

					add_settings_field(
						$license_key_key_name, // id
						__( 'License key', 'inventory-presser' ), // title
						function () use ( $option_name, $current_value, $license_key_key_name ) {
							?><p><input type="text" name="<?php echo $option_name; ?>[<?php echo $license_key_key_name; ?>]" class="regular-text code" id="<?php echo $option_name; ?>[<?php echo $license_key_key_name; ?>]" value="<?php echo $current_value; ?>" /></p>
							<p class="description"><?php printf('%s <a href="https://inventorypresser.com/">https://inventorypresser.com/</a> %s', __('Obtain a key at', 'inventory-presser'), __('to receive plugin updates.', 'inventory-presser')); ?></p><?php
						}, // callback
						INVP::option_page(), // page
						$settings_section // section
					);
				}, 20 
			);

			$hook_parameters_count = 'license_key' == $license_key_key_name ? 3 : 4;
			add_filter( $sanitize_values_hook_name, array( __CLASS__, 'sanitize_and_activate_license_key' ), 10, $hook_parameters_count );
		}