The related parameter specifically shows related users after the web intent has been posted:
Suggest additional Twitter usernames related to the Tweet as comma-separated values. Twitter may suggest these accounts to follow after the user posts their Tweet. You may provide a brief description of how the account relates to the Tweet with a URL-encoded comma and text after the username.
This does not add the @handles to the Tweet body. It just shows a list of users that were related for the user to follow after posting the Tweet via the composer. I’ve just verified that this is working as intended for me.