You can use the general polygonal load or rectangular load to define the load geometry. The load magnitude will be variable (non-uniform). To define a load magnitude which varies over the area of the polygon, select the Variable checkbox and select the Define button. You will see a dialog which allows you to define the load magnitude at each vertex of the polygon.
The Fill load, which is defined in the same way as a polygonal load, can also be defined as variable.
Define Load dialog
Define Load on Each Vertex for a non-uniform pressure distribution