[rdiddly]

You've taken a very specific question/situation and vastly generalized it to "How do I manage my manager?" The answer is most likely, "Avoid doing it if possible," but why don't we instead begin (and maybe end) our manager-management course with the easier question, "How do I deal with the fact that my manager told me not to speak at a conference?"

If you're relying on the company for the conference in any way -- for example, they're paying your way, or you're going to talk about their technology (and even if you built it for them, it's usually still theirs) -- then the manager has the right to approve or disapprove. If neither of those two elements are present, and there are no others that I haven't mentioned, (or you can quickly make it so), then the manager needs to be told gently to fuck off, and why.