When you land on a page with a password field, you'll see a small True Key icon in the top right corner of your screen. (You'll need to be signed in to the True Key app when using this feature - but the icon will appear on all supported browsers and apps.)
Tap the icon and a small banner will appear at the bottom of your screen. Choose the login you'd like to use, tap on it and the app will automatically fill in your username and password for you.
If the login isn't already in your True Key profile, you can add it by tapping 'Create New Login' or 'Use a login you have already saved'. You'll be directed to the True Key app so you can add and manage logins (and use them for Instant Log In once they're added).
If needed, switch back to the regular keyboard by clicking on the keyboard icon.