How to round a corner in After Effects without rounding the others?

Hi,

I would like to create a rectengle with a rounded corner however I'm only needing one corner to be rounded and the others to remain straight.

I would like to use this rounded edge rectengle for an animated lower third using expression code in the size tab thats found in the shape layer under Contents > Rectangle Path > Size.. here is the problem in order for the Size option to appear the shape has to be parametric shape which it starts off as.. however altering the shape in any way seems to convert it to a Bezier Path and I lose the option to add the expression code to the Size tab .. is there a way around this?