Unfortunately, WikiTree isn't set up to let people simply subscribe to a profile in order to add it to the watchlist.
If you want to add a profile to your watchlist, submit a Trusted List request for that profile.
And if you want to add a line of relatives and ancestors that are managed by another specific member, in your request you can mention the other profiles you are interested in, so they can add you to a bunch of a profiles at the same time. If you are bulk-added to a bunch of profiles, do check your watchlist to see if you were added to "too many" profiles (sometimes a person will generously add you to all profiles they manage, which likely includes many people you aren't interested in). There used to be a mechanism for seeing recent additions to our watchlists, but I can't find it any more...