If you haven't been officially diagnosed, I would wait to add it until it's official. That being said, you can call to add it to to your ADRs after the fact if you need to. Now, my older one is allergic to peanuts and tree nuts, and I always have her allergies on our ADRs + I make sure it's noted when I check in for a meal and I also speak to our server (and sometimes the chef) about it once seated. And I've had to add them by phone a couple of times after the fact, so it shouldn't be an issue.
All this being said, I don't think I'd put it on there if you're not certain. Case in point, I have allergic sensitivities to certain shellfish...specifically, bivalves. I had one trip where I decided to list shellfish allergy on everything, since technically, this falls under that and I will get sick if I have things like clams, mussels, scallops, etc. and it's not uncommon for some seafood dishes to involve broth/stock from clams. I remember having drama with the chef at CRT because she refused to serve me shrimp since the allergy info on my ADR listed shellfish. She didn't want to hear about the breakdown of shellfish and how you can be allergic to some categories, but not all. I was told prior to the trip that it didn't qualify for Special Diets' attention because it falls under a common allergy. So, I had to skip the shrimp at that meal because of this and it was not a pleasant interaction with the chef. After this, I've just used my best judgement when ordering.