itch.io

You should be able to open the .pdx file (it's really just a zip file I believe), edit the pdxinfo file to add the necessary fields, and package it back up into a .pdx. Then adding the game via the Sideload page in your account would work.

FWIW, I believe you can directly add the game via USB without a valid pdxinfo file if you'd rather go that way.

The pdxinfo file needs additional fields filled out in it for sideloading via a user's play.date account. More info here: https://sdk.play.date/#pdxinfo. The docs don't indicate which fields are required, but I believe it might just be bundleID. I would recommend included that as well as name, author, and version at a minimum.