If I understand your question correctly, then you may have to use the Freeform tool to create your desired shapes. I don't see any 'shape cutting' tools in PPT to create a shape by subtracting pieces from it (which is how this task is normally performed in a vector program):

Draw a square
Draw a circle on top of that square
Then use the Freeform shape and begin tracing one of the corner spaces (the shape that's produced from square minus circle)
Right-click your new custom shape, select "Edit Points"
Right-click one of the two corners that will connect the curved portion of the shape,(select "Corner Point"), repeat this with the other corner



Your cursor should become a square with points, the points on your new custom shape will have protruding handles

You can now drag the handles to create the curved portion of your shape.
Once your new custom shape looks like the custom corner you're trying to copy, you can then duplicate it and rotate the different pieces to make up the other corners. Fill them with whatever color you want.

Is that what you want?