I am confused, shall I activate my trial license on my web account page, or on the Immerse Gamepack application?

You can use either the Immerse Gamepack desktop app or Immerse Gamepack Web app to activate the trial account. You can download the Immerse Gamepack Installer from here. Once installation is complete, select Launch Immerse Gamepack Final Fantasy XIV to activate your license or free trial. 

or if the game prompts you to the web application, you can complete your profile on your browser. Please enter your email and activate your trial.