I'm not sure exactly what you mean by garden boxes but in general planting for chickens needs to be either seen as disposable (sow something that grows quickly, let them trash it, then either let it regrow or sow a replacement), or protected in some way (e.g. mesh around or over something so they can only nibble what grows through), or large, vigorous and well established.
The more (both in quantity and choices) you can provide, the less likely they'll be to completely destroy one plant - but that's not a guarantee.