Use static class Method in interface type class in Java

How to use public static class method in Interface class?

for example:

public class Var {
    private static String api_key;

    public static String getApi_key() {
        return "Authorization: " + Var.api_key;
    }

    public static void setApi_key(String api_key) {
        Var.api_key = api_key;
    }
}

then i want to get method getApi_Key(); //return String 2773167988874443412478834 to be usable in interface class

public interface VerifyClient  {
    String api = Var.getApi_Key();

    VerifyResponse verify(api);
}

iv'e tried many approach but it wont work. so how to solve this?