If you are a regular M-Files user, you can add properties while filling in the metadata card. You can select the Add property label in the metadata card to add a new property for the object. Please note that this property is only added to this particular object, not all objects of the class.
If you want to add a property to all documents of the class, go to Classes in M-Files Admin, right-click the desired class and select Properties from the context menu. You can add default properties for the class by clicking the Add... button. If the desired property cannot be found in the list, you need to create a new property definition (see Property Definitions).