Add a link to a menu through the menu's add-link form (/admin/structure/menu/manage/<menu>/add), by the menu's machine name ("main", "footer"), with a title and a path or URL.
Examples
When I add a menu link titled "Sample" to "/sample-article" in the "main" menu
And I add a menu link titled "Home" to "/" in the "main" menu
When we add a menu link titled "Drupal" to "https://www.drupal.org" in the "footer" menu
And I add a menu link titled "Log in" to "/user/login" in the "account" menu
When I add a menu link titled "Node one" to "/node/1" in the "main" menu
Pattern
When(/^(I |we )*add a menu link titled "([^"]*)" to "([^"]*)" in the "([^"]*)" menu$/, …)
Part of @webship-js/drupal-core-steps (github.com/webship/drupal-core-steps), a step pack for Webship-js 2.0.x.