To implement the Pop-up you would need to add the script to your Master template following the steps below:
Go to "Settings" on your Umbraco admin page.
In the Settings menu choose "Templates" - your "Master template".
Using the code editor add the Pop-up script to the <head>.
If you do not have a master template already you can create one following Umbraco guides. You can read more about master templates here and templates in general here.