City selector with free text

I'm creating a web component used to select the name of a Belgian city. Basically, it's a simple input and when some text is entered in it, it display the list.

However, it must be possible to type the name of a city outside of Belgium. The issue is that, for internal reason, we don't and can't have a list of all city in the world, we only have the Belgian ones.

So the expectations are that a user must select a city if the desired city is in Belgium. So he can't just type "Braine l alleud" and get away with that, he really needs to select "Braine-l'Alleud" because the item is bound to some JavaScript instruction that select the city. But, if the desired city is outside of Belgium, then the user must be able to type something freely and get away with it.

So basically, I want the ensure to be force to select a city if it's in Belgium but be free to type a city name if it's outside of it.