You can find your account's ID through several methods::
1) If you have access to the account in the game, then you can look up account's ID via profile section in the game itself:
2) If you do not have access to the account in the game, then you can find ID of the account via profile page at the game's webstore: https://store.gaijin.net/storefront.php?category=WarThunderMobileStore
Please note, in order to access account in the store, you need to use the same authorization method you use in the game: