Beginner Question: Making a Recess From a Chord in Circular Face

[Natustell]

I am trying to model this (sans the cable connection):



How do I create the straight recesses in the top of the cylinder (highlighted in red).

I assume that I need to create two parallel chords first to separate the circular face into 3 regions, then extrude the middle region to reach the full height of 9.20 mm. This is not working because I can't make lines that are actually attached to the face, as shown here:

 

[Cadasio]

Its really hard to tell from you image but it look as though you may have started your sketch wrong. I would start a sketch on the top face of the cylinder shape, draw my two lines then cut downwards

Top view:




Iso(ish) view