Custom property view will manage all custom properties (user defined properties) in the plot. You can click the "Custom Property" button on the right toolbar, or press Ctrl+J (Command+J in Mac OS) to show/hide the custom property view.
You can click the button to add a new custom property. You can then specify the details of the property in the bottom editor.
Please notice the property has different syntax in TEXT and EVAL parsing modes, and you can preview them here.
You can also choose the property type from available data types.
Once you define the custom property here, you will be able to insert them into expression.