How to enter non-existing ICAO code?

Whilst creating a new, non existing airport, the SDK editor asks for a valid ICAO code. How can I avoisdd that and create a code of my own?

You should be able to add whatever you want, assuming it doesn’t already exist in the real world. 4 chars is all you need. What I do is I find the names of real airports in that vicinity and base my fictitious code using that as a base.