Your unique Facebook User ID for Golf Clash can be found in the 'Apps and websites' section of the Settings page on Facebook.

In order to access this, you must login to the Facebook app through your mobile device or through the website on a computer.  Click your portrait/profile picture, select 'Settings & privacy' and then 'Settings'.  On the left hand column, locate and select 'Apps and websites', this can usually be found further down.Find Golf Clash and select 'View and edit'.  From there, near the bottom of the page you should be able to see some numbers, for example: 10218940150942477.

You can also find steps for locating your Facebook ID via this link: