KiCad’s drawing capabilities are quite limited. It’s not a mechanical CAD program.
One solution is to create a pad layout in a mechanical CAD program, and then import it as a .dxf file. FreeCAD, together with the “StepUp” plugin has a lot of custom capabilities, but it has a steep learning curve.
A simple intermediate solution is to draw a few simple shapes on any layer, and at least one pad on a copper layer, then select all those parts and make a custom pad from it with the popup menu.
It needs at least one single “real” pad because that is used as the attachment point for PCB tracks.
You can also draw an arc segment for the inner round part, and then roughly overlay a coarse polygon, but is it really important? Just a coarse approximation of 3 to 5 line segments of the arc is good enough, and there is no reason to spend more time on it.
I would make the arc’s in the corners a bit bigger though. Pads are used for the solder stencil also, and round ed corners help with releasing the solder paste from the stencil.
[Edit:] Ah, indeed. I forgot to add the link that BlackCoffee did not forget.