Submit the contact form with its button (.s_website_form_send) and wait for the answer of the site: the confirmation, or the errors the form shows. Assert the outcome in the step that follows.
Examples
When I submit the contact form
When we submit the contact form
And submit the contact us form
When I send the contact form
And we send the contact us form
Pattern
When(/^(I |we )*(?:submit|send) the contact(?: us)? form$/, …)
Part of @webship-js/odoo-steps (github.com/webship/odoo-steps), a step pack for Webship-js 2.0.x.