How to Allow Custom Search Query Widgets for Post Types
It is possible as the admin to activate this option to allow custom search query widgets for post types.
*Before activating this functionality bear in mind that users can easily break their sites adding a value here. If they are not a developer, it is recommended to keep this setting disabled.
Please see the following points mentioned on the image shown below to activate the custom search query widget option:
Step 1: Navigate to Settings >> Advanced Settings >> Search for the words "allow custom".
Step 2: Click on the button to turn on the functionality.
Step 3: Save the changes.
After the option has been enabled go to Content >> Edit Post Settings >> Select the Post Type you would like to add the custom search widget and click on Edit:
Move to the "Search Results Design" tab and scroll down until the Additional Settings option becomes visible.
Finally, enter the desire custom widget in the corresponding field: