You cannot add negative keywords at the platform level, even though this is even more important with a fully automated campaign like Performance Max. The only way to add negative keywords is to go through your Google Rep. Hopefully, this will change, but there is no ETA. And, to add insult to injury, if you do not have a dedicated Google rep, you have no way to add negative keywords to PMax. Support will not do this. So this is a big red flag for smaller advertisers and agencies.