Hi! PICO-8 has the export command, which lets you upload your games as HTML5 or executables. There's this tutorial by Lazy Devs which I haven't watched but you might find be useful:
The TLDR is that you can do "export mygame.html" to get two files, a .html file and a .js file. All you have to do is to rename the .html file to "index.html" and zip the two files together. Then when uploading to itch.io mark your game as runnable from the browser and you should be all set! Feel free to ask more questions if you have them!