Was your laptop was made in the last 5 years, and when you open the graphics settings (right-click the desktop and select the graphics options) can you pick which programs use which video card? If so, then you can't do it. These laptops are set up so the Intel video card is always driving the screen. The dedicated GPU acts as a co-processor. When a game uses it, the dGPU renders a frame, then sends the completed frame to the Intel graphics card. The Intel graphics card then displays it on the screen. (Vsync is basically on all the time, with the two GPUs acting as the two vsync screen buffers.) If you disable the Intel video, you get a black screen because nothing is driving the screen. The dGPU is not physically connected to the screen, so has no way to display to it directly.
There are a handful of laptops which let you choose which video card drives the screen (the dGPU *is* physically connected to the screen). These laptops will have a BIOS setting which lets you select which video card is active. On laptops older than 5 years, some of them had a physical switch to let you select GPU. Likewise, a handful of laptops are set up so the Intel card drives the screen, while the dGPU drives the HDMI or displayport out. These are useful in the handful of older games which assume computers only have one video card and so are always stuck running on Intel video despite your graphics settings. (They search for a video card, find the Intel since it's driving the screen, and stop searching for another GPU.) You can't play these games on the laptop screen with the dGPU. But if you hook up an external monitor, you can play them with the dGPU.
But these laptops are comparatively rare since the manufacturer has to go to extra trouble to design it this way. The vast majority of laptops (like 99%) are designed with the Intel video card controlling the screen and video out ports, and the dGPU acting only as a coprocessor.
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