Simply tell the community manager that you are a Croissant member and then check in with the Croissant app or website. They will be notified of your arrival via their email and dashboard.
Please note that some spaces may ask to see an ID for security purposes and in order to enter the building. Check the "arrival instructions" for each workspace on their listing page, as sometimes there is extra arrival information for the space or building that will help speed up your entry.