There are two different options to use Turnitin® in Moodle:
Plagiarism Plugin - When creating a Moodle Assignment or a Forum, you have the option to enable the Turnitin plagiarism plugin. The plugin allows to integrate the Originality Report settings into standard Moodle activities.
Turnitin Assignment 2 - Turnitin's Moodle Direct integration activity module is a standalone Moodle activity that allows the teacher to create multi-part assignments that are automatically checked for plagiarism. For each assignment, an Originality Report is created.
See Moodle Docs for more details about using this activity.