You’re pretty close, you just need to replace the = with a :. You can use an object type literal (see spec section 3.5.3) or an interface. Using an object type literal is close to what you have:

var obj: { property: string; } = { property: "foo" };

But you can also use an interface

interface MyObjLayout {
    property: string;
}

var obj: MyObjLayout = { property: "foo" };