Exactly you have to put your own application.i tried on wordpress 5.3 its not working even on twitter and linked in its not working it only works whe you put your own app
Remember that it is a nulled you cannot use the fs poster app since they have to add your site to the url callback.
This is done by the guys at fs poster when you buy the plugin.
But in the case of nulled it is better to just create an application.
It's too easy.