I'm very new to SharePoint and stuck on below:

I've a site with loads of events and all of those events uses 1 registration form. Every time a user wants to register to an event they have to navigate to the actual event page and then click 'Register' which open up the form in a separate browser window. 

What I want to do is to redirect the actual event page to a 'Thank you' page once the form is submitted.

My problem is, as I'm getting the user to fill the form in a separate  window how do I get the actual event page to be redirected which sits at the background.

hope this makes sense.

thanks