For those claiming their internal transfer were blocked, what your manager told you about why, maybe you are important to the work, cannot afford to lose you?
Some groups would not take your interest seriously if you do not do online applications first to start, e.g., take a look at your resume and talk to you informally. Why do they waste time on anyone wanting to just talk? Doubtful the system sends the notifications to current managers about your applications. It was changed years ago to no notifications till the offer was to be made.
Ideally, you interviewed with the hiring group and got some unofficial offer, then you talk to your manager about your new interest. Then the two managers talked, if your manager does not block you, the offer will be made official, and happy you go.