Populate custom Managed Properties in Build Your Query editor
You can build custom Managed Properties, but when you use the "Build Your Query" editor, the Property Filter drop down doesn't include your custom property and you have to manually type it into the Query Text.
Steps to reproduce:
Create a custom Managed Property in SharePoint Admin > Search
Example: CheckedOutUser linked to ows_CheckoutUser
Make it Searchable, Queriable, and Retrievable
Reindex your site
Wait for next crawl (4-8 hours)
Create search webpart
Edit the webpart's search query
Select the Property filter dropdown in advanced mode and "show all managed properties"
Your custom property won't be listed (no CheckedOutUser)
You can, however, manually type this into the Query text below to make it work