Login to your Facebook account. From the homepage, click your profile. On the address bar of your browser, highlight the complete Facebook URL (should look like this: www.facebook.com/your_fb_id), right click the highlighted URL and click copy.
What is the Facebook profile URL?
Click your profile photo or "See your profile" under your name. Click the "Profile Settings" tab (looks like three dots). Scroll down to "Your Profile Link" and click "Copy Link" to copy your complete Facebook profile (should look like this: www.facebook.com/your_fb_id).