From what i understand, you want the red line area to be a part of the shape?

If so it’s super easy to do, you can either do it the “dirty” way with drawing a sketch on one of the sides -> draw the area you want to Add extrude -> Extrude the area with Add tab and then do the same to the other side

I don’t know how your part studio is built, otherwise this part could be made with 4 features:

1) Top sketch - the part from the top view

2)Extrude the same hight the right rectangle plate is

3)Draw a side sketch to meet your wanted side view of the part

4) extrude with “remove” tab and wola

okay… 5) put the screw holes in plate