Pre-filling the message
parameter does not work any more – Facebook removed that a long time ago.
You are not supposed to pre-fill any text when sharing on Facebook – the message should be typed in by the user themselves.
If you really want to do this, you have to make a post via API in the background – which of course requires the user to connect to your app and give adequate permission. And even then you are not supposed to post a message that the user did not type in themselves into a form somewhere – and Facebook easily figures out if your app posts the same (or largely similar) messages for different users, and that will limit visibility of your posts drastically, and could even lead to your app getting blocked.