Refer to this post: Explicitly choosing which GPU to use - Community / Community & Industry Discussion - Epic Developer Community Forums (unrealengine.com).
In short, use command line argument: -graphicsadapter=2 (search “rhi” in your Output Log to find the GPU adapter number). If GPUs are from different vendor, you can also use: -prefer{AMD|NVidia|Intel}. In my case, I prefer AMD iGPU to Nvidia GPU for better cooling on my laptop in the summer. So I use -preferAMD.