https://helpdesk.lastpass.com/lastpass-mobile/ios/

Unfortunately, it won't automatically fill in the username and password for you like the desktop browser extension will. You'll have to click on the share button and, assuming you've set up LastPass per the directions in that link, click the LastPass button. It will then fill it in for you. Sounds like Android does a much better job of handling this. Fortunately, once you login to a site you shouldn't have to do so again for a while.