You've jumped all the way to trying to micromanage your boss, which is inherently a politically explosive undertaking, but where you actually should be is just asking much more straightforward questions: why don't they want you to give the talk? Maybe they're trying to sabotage your career progress, but maybe there are legitimate concerns about the content of the talk (e.g., exposing secret sauce internal methods to competitors in an extremely public way) or maybe they just don't want to grant the budget to send you.
To expand beyond this, generally the way to "manage" your boss is to get to the point where they don't have to manage you. While you won't formally or hierarchically be on an equal footing with them, informally you will be because at that point you can have a continuous ongoing relationship based on mutual trust.
Yes, most commenters here are missing a legitimate concern about sharing confidential information in a public forum. If the talk is "How we do X at Y Inc" there's a legitimate concern; if the talk is "Learning to Love CSS in JS" then the manager may have ulterior motives.
My advice to OP: ask to walk through your presentation deck with your manager, both to get feedback and buy-in. Managers will be more invested in your success if they feel invested in what you're doing. (Suggestion: offer to plug your company's job openings with your manager's contact info.)
To expand beyond this, generally the way to "manage" your boss is to get to the point where they don't have to manage you. While you won't formally or hierarchically be on an equal footing with them, informally you will be because at that point you can have a continuous ongoing relationship based on mutual trust.