The short answer:   you cannot;  it's not the way this model is designed.  The nVidia GPU is a secondary to the Intel GPU -- only the latter has a physical connection to the outside world. There are systems such as the XPS 17 and some Alienware systems that have firmware-switchable GPUs -- it's not a feature of the system you own.  
I am using Dell Inspiron 7591 which has intel integrated graphics and Nvidia GTX 1050 GPU. I recently purchased a Dell S2421HN monitor and I am connecting it with my laptop using an HDMI cable and I can't get my external monitor to use my dedicated GTX 1050 GPU. Is there any way to use the dedicated Nvidia GTX graphics in the external display?
How to use my dedicated nvidia gpu with my external monitor.