I solved :)
this is the reason why it did'nt worked before:
In my Form definition the name was "zanzibar_backendbundle_countries".
public function getName()
{
return 'zanzibar_backendbundle_countries';
}
So to bind a request to this form the json should have looked like this:
{"zanzibar_backendbundle_countries": [{"description": "Japan"}]}
Since I wanted it to be something like
{"id":1,"description":"Italy"}
I had to remove the name from the Form:
public function getName()
{
return '';
}
In general if you want to post a json with a placeholder like
"something":{"key":"value"}
your form name must be exactly "something"