No problem, and yes, I already understood the requirements.
Should this be done in another way?
I would say so. Otherwise, as mentioned in my previous reply, it would expose the entire client and project list to anybody in public.
There was a similar question, but it refers to the login rather than the signup, where I described 4 different options.
What’s not 100% clear: is that signup entirely open for anybody, so anybody can signup for any client and/or project?
And why does the user need to choose client/project at signup in the first place? If a user signs up for multiple clients/projects, does that require a separate signup with different credentials, or can he re-use the same for all clients/projects?
Maybe the linked thread above already answers the question or gives you an idea of another approach.