Thanks all! This particular roast doesn't come with a seasoning packet, just the marinade. I'll follow the bag's (and your instructions).
Secondary question.... can it be made in an InstantPot or is low & slow in the oven/stovetop the best method?
Yes, it can be done in the instant pot.
https://damndelicious.net/2019/03/06/instant-pot-corned-beef-and-cabbage/For an improved texture that slices better, I like to cook one day and then wrap in foil and weigh down and refrigerate overnight. I then reheat the next day in the oven while wrapped in foil. I might add a glaze the last 20 min wunrapped depending on how I'm serving it.