I created the "Custom Property Test" property using CustomWindowsProperties from https://github.com/Dijji/CustomWindowsProperties.
I was able to add the new property as a column in Everything.
At first I didn't see how to get any data into the new property, or how to get the data to show up in the Everything column.
I got the custom property to appear in the File Explorer Details Pane using FileMeta at https://github.com/Dijji/FileMeta/releases/tag/v1.6. I went into the FileMeta Association Manager and added the new property to the profile of properties for TXT files.
I entered some data into the custom property. I restarted Everything and there is the data in the column.
Unfortunately Dijji, the author of FileMeta, passed away in May 2021, and no one has further developed the program yet.