monitor acting as a second screen for broken screen macbook air 2020

I accidentally cracked my macbook so i decided to not waste money so i bought stuff for my macbook like HDMI. So i connected my macbook to my monitor and when i turned it on, the monitor acted as a second screen for my macbook even though i want it to display what my main macbook screen is doing. I cant change it in the settings because the screen is broken. What should i do? The monitor only shows a black screen and the mouse that can move between screens.