New sites should come with this automatically but if for some reason your site doesn't have a search option in your Reviews search results and you wish to add it, please follow these steps.
1. Go to Content >> Edit Post Settings.
2. Click on the Reviews post setting.
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, you will be able to do searches in your Reviews search results page.
If you have more questions, please submit a ticket HERE.