Firstly, in your project you need to confirm that the project is configured to permit synchronization with external calendars. Go into the Project Options screen and open the Options section.
The allow external calendar option must be set to yes. If not, contact your System Administrator to change this setting.
You will see a screen similar to the below. Copy the link provided by clicking Copy Link.
Open your calendar. We'll use Google as our example. Click on Other Calendars to produce the following dropdown.
Select Add by URL. When the dialogue box opens, paste the URL in the empty box. Then click Add Calendar.
Be patient while the calendar is retrieved by Google, this can sometimes take several hours.