I was looking for exactly the same thing a few months ago. Most thesis services focused on delivering content but didn't really care whether it sounded like the student. The better companies asked me for old essays, discussion posts, and even research proposals.
The voice-matching process took a few days because they analyzed my writing before assigning someone. The final result wasn't identical to my style, but it was close enough that my supervisor didn't suddenly notice a huge difference.
One suggestion: don't only upload your highest-graded papers. Include average assignments too. If your thesis suddenly sounds significantly more advanced than everything you've written before, that can raise questions. Consistency matters more than sounding impressive. I'd also ask whether the service has experience with your discipline because citation expectations vary a lot between fields.