I see.  all along I thought the question was the black antenna installation (aux) which intrigued me. you are actually asking about the main antenna (white) connected to SMA rod at back. Re: I am still puzzled that the location of the main antenna is a round indentation with a narrow slit.  no more puzzle.  the round indentation is a piece of detachable metal cover.  the narrow slit is meant for user to stick in a flat blade screwdriver to twist off the detachable metal cover.  you do not drill through it.  this is similar to how you would remove the detachable cover for the adjacent optional video port  Dell instruction: To remove the dummy metal bracket, insert a flat-head screwdriver in the hole of the bracket. Push the bracket to release the bracket, and then lift the bracket out from the system. [IMAGE]    
I recently installed an Intel 8265NGW Wifi and Bluetooth M.2 card into my Dell Optiplex 3050 MFF. (The product photo above is not correct). The card came with two internal antennas (main and aux). 1. There seems to be no particular place to attach the internal antennas. All the online instruction videos I can find focus on the card not the placement of the antennas. Where do they go? 2. Since it is a metal box, I suppose it would be better to attach an external antenna, but the chassis does not appear to have a suitable round hole for the socket. In the corner where I would expect the antenna to go, there is a round indentation with a thin slot. Is that it? It does not appear easy to knock out.  
How to fit internal and external antennas to an Optiplex 3050 MFF?