record MyRecord(int a, int b, int hash) {

    MyRecord(int a, int b) {
        this(a, b, Objects.hash(a, b));
    }

    @Deprecated
    MyRecord {
    }

    @Override
    public int hashCode() {
        return hash;
    }
}

That said, I consider this an odd usage of records and would never do it myself. If I was convinced that I needed to precompute hash codes I would go with an old fashioned hand written value class.