If you have a Moodle account but you wish to use Turnitin® without integrating the two platforms, you can access the plagiarism check service at https://turnitinuk.com/login_page.asp.
Since you already have an account automatically created together with your Moodle account, you simply have to click on Forgot your password? to reset your password and subsequently access the service independently.
If you do not remember the email address for your account, ask the Turnitin® administrator to look up for your address.
Once you are logged in, you can add classes and assignments and eventually upload the files that need to be checked.