Tag the post in the editor sidebar. A tag that does not exist is created by WordPress when the post is saved.
Examples
When I tag the post with "webship"
When we tag the post with "release"
And I tag the post with "testing"
When I tag the post with "news"
And we tag the post with "wordpress"
Pattern
When(/^(I |we )*tag the post with "([^"]*)"$/, …)
Part of @webship-js/wordpress-steps (github.com/webship/wordpress-steps), a step pack for Webship-js 2.0.x.