How can I add a semi-cylinder to the side of this face?

i am pretty new to OnShape (a couple days new) and modeling isn't my domain of expertise. However I designed this part in the image attached, but am struggling to figure out how I can add a semi-cylinder to the orange highlighted face.

I did so easily on the clips below the base since I could sketch from a visible side, add an arc and extrude. However in this case, there are no immediate visible sides for me to click on and extrude an arc.

What's the proper way to do this?

Thanks in advance!