Thanks for getting in touch Gijs. I would recommend doing a collaboration batch for this beer rather than a gust beer so that Breww will take the duty into account. You can manage the batch as you would with your own beers but Breww will take into account that this is a collaboration rather than brewed in house. I’ve included some more information on this below for you.
I hope that helps but let me know if you have any questions on this.