![Image](https://i.imgur.com/UMbSKFg.jpeg)
To bring up this dialog, click File > Info > click the tiny down arrow next to "Properties" > Advanced Properties > Custom tab. I filled in the Name box with "Custom", and the Value box with "test", then the Add button. The Property then appears in the list of in the bottom part of the dialog box. Could Everything use this "Custom" property? Unfortunately, after adding this property, and saving the file, the "Custom" property still doesn't show up in the File Explorer Details pane, or the file properties page Details tab.