Open the Wireless Display app on the laptop you're using as a second monitor On your main PC, press Windows + K to search for wireless displays. Select your laptop's name from the list of options.

I urge those considering a second monitor to try virtual desktop tools first and plan exactly how you’ll use that second or third display! 5 Reasons Why You Don’t Need a Second Monitor on Your ...

Using a laptop as a monitor for another computer allows you to not only mirror your computer's screen on the laptop but also extend it, meaning you can show different content on both screens.