New sites should come with this automatically but if for some reason the site doesn't have a search option in the Reviews search results and the admin wishes to add it, please follow these steps.
1. Go to My Members >> Member Reviews.
2. Click on the Member Review settings.
3. Click on the Search Results tab.
4. Scroll down to expand the "Additional Settings" and make sure that the Sidebar To Use is Post Search Results.
5. Select in the Sidebar Search Module the Bootstrap Theme - Module - Reviews Search.
With this setup, it's possible to do searches in the Reviews search results page.