Yes, you are required to use TurnItIn through your dissertation process in accordance with your school's requirement.

Capella’s use of Turnitin is designed to combat plagiarism, which has become a significant problem in institutions of higher education.  Use of plagiarism detection software in general, and Turnitin in particular, is increasingly common in educational institutions.  According to Turnitin, 10,000 institutions use the service, including more than 60% of top colleges and universities.     

Capella University’s contract with iParadigms specifies that papers submitted by Capella instructors and learners are retained in Turnitin’s Source Database solely for purposes of using such papers as source material to detect plagiarism of such papers by other students at Capella.  Capella’s agreement also requires iParadigms to comply with FERPA to protect learner privacy.