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 esc_attr( $option_name ); ?>[<?php echo esc_attr( $license_key_key_name ); ?>]" class="regular-text code" id="<?php echo esc_attr( $option_name ); ?>[<?php echo esc_attr( $license_key_key_name ); ?>]" value="<?php echo esc_attr( $current_value ); ?>" /></p>
								<p class="description"><?php printf( '%s <a href="https://inventorypresser.com/">https://inventorypresser.com/</a> %s', esc_html__( 'Obtain a key at', 'inventory-presser' ), esc_html__( '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 );
		}