In blender I pressed the N key and then on the right side I selected Tools > Workspace > Custom Properties > then clicked on the + New and gave a name to the new property.
Same I did on the side in the Object tab also added new property.
Then saved and in the menu: File > Export > FBX (.fbx) and in the export window I checked the checkbox: Custom Properties.
but then when dragging the file to the unity editor I see the model but not see the new propety.
and in unity3d editor after importing: I don't see the new property Fill Amount: