You have two options to log in with Facebook: create a new account with an existing Facebook profile, or link your profile to a Facebook account once it's already created.
If you have created your account using your email address and you want to link it to your Facebook account, select the 'Friends' icon. It is located on the lower left corner of your main screen, represented by a two-people icon.
There, you'll see the option to link your account. You will be asked to confirm that Word Crack wants to use Facebook to sign in. Once you confirm this, you will be redirected to Facebook where you will have to confirm that you want to link both accounts.