Unless you’re a devious supervillain, there’s no reason to keep your goal a secret from your audience or save it for a big reveal at the end of your talk. Presentations should not be mysteries or James Bond movies. Besides, if you’re less than forthright with your audience they’re likely to see right though you.
When they do, chances are that they’ll not only resist doing what you’re asking of them, but that they’ll be unlikely to ever trust you again. Trust is very difficult to rebuild once it’s been broken, so it’s always a good idea to be as clear about what you want and as transparent about your motives as possible. Presenting is all about persuasion, but lying, misleading, or covering-up aren’t effective long-term strategies.
Your audience will appreciate your honesty when you’re open with them and you’re much more likely to build a productive long-term relationship when it’s built on trust.