Head to your Google calendar website. This can usually be found at calendar.google.com. Ensure you are logged in to your correct Google account.
On the left hand side there is a list of all your calendars.
Find the one you would like to connect to Event Calendar App, and click the three dots next it it. Then click 'settings and sharing'.
This will open a new page. Scroll down until you find the Integrate calendar section. Below this you will see a section that says Secret address in ical format. Copy this url to your clipboard.