Choose Home tab→Reference group→More Planes→By 3 Points button.

Select a point to define the reference plane's origin.

Select a second point to define the positive x-axis of the reference plane. The distance between point one and point two determines the width of the reference plane.

Select a third point to define the positive y direction of the reference plane. The perpendicular distance between the x-axis and point three determines the height of the reference plane.