Select the face as shown, then click Base-Flange/Tab
(Sheet Metal
toolbar).

A sketch opens on the selected face.
Sketch a rectangle, making one edge of the rectangle
coincident to the upper edge of the edge flange.

Dimension the rectangle to stick out 20 from the edge flange and be 40 long.
Add a coincident relation between the midpoint of one of the long lines of the rectangle and the Front plane:
The tab is added to the part. The
SolidWorks software links the thickness of the tab to the thickness of
the base flange.
