I do not have any images to show you . We at our work place also have created a dashboard which will be used as a gate portal.
These are the steps we follow:
1. Create an excel or Google Sheet/Excel Online with your dashboard names and tableau server links. Better to use Google Sheet/ Excel Online as it is easy to add or remove dashboards and links based on new dashboards created at your organisation.
2. Use this data to create a dashboard using the links field in URL actions. - URL Actions
Hope this helps