Open the "Elements" menu and its "rect" sub-menu. Click on the icon "Rectangle" to activate the function.The icon is framed by a dotted line to show that it is activated. The function displays the message "Digitize the rectangle by top left and bottom right corners."

Click the first point

Click the second point, you can point each point separately or continuously as with a pencil.

Drawn rect is selected and the floating menu shows available editors to apply on it.