Once your game is compiled, you should end up with a z5 or z8 file (if compiling to a z-code file) or ulx or gblorb file (if compiling to Glulx). I suspect you'll have a gblorb file. Just upload that file to your itch.io game page and provide instructions on how to run it using an appropriate interpreter. See ifwiki.org for a list of recommended interpreters for various platforms.
In the longer term, you can look into providing a browser playable version using Parchment. Instructions to do this are available in the Inform 7 help, but the templates and other files that it uses are out of date. Do a search for Parchment on intfiction.org and you should find some more up-to-date instructions.