Unless I’m missing something in your question……”M” is your friend.
Set your ISO to something other than ‘A’ (auto), such as ‘200’.
Set your dial to ‘M’ (manual).
Now you can set the shutter speed and f stop to obtain the exposure you seek.
On the dial, ‘P’ is auto exposure, the camera will set the aperture and shutter speed for proper exposure. ‘A’ is aperture priority, allowing you to set the f stop for depth of field and the shutter will automatically adjust for proper exposure. ‘S’ is shutter priority, allowing you to set a specific shutter speed to stop action and the aperture will automatically adjust for proper exposure. ‘M’ allows you to manipulate both the shutter and aperture (f stop) to set the exposure of your choice (provided you take the ISO setting off ‘Auto’.