Please follow these steps to find your FPL ID.

Initially, log in to your account on https://fantasy.premierleague.com and then proceed by selecting the Pick Team tab, which is illustrated below:

Once here you will see on the right there is a link titled Gameweek history ➜ . If you click on that, when the page reloads look in the address bar you will now see your ID, in the example below it's 952757.