Fritzing doesn’t really have a mechanism to create irregular shaped copper areas other than ground fill. There are ways of working around this to create such an area. Here is a tutorial on making a capacitive touch pad which is a solder mask covered copper area like you are looking for. How to Make Capacitive Touchpads in Fritzing
You could also use the Logo part in the core parts bin to import an SVG of the shape you want and then use that as your copper area.