If you are going to start an adult website, you do not need any DMCA agent, it is not required and if you will get any copyright notices, 99% of hosting providers will comply with these notices, no matter whethere they are based.
You can do everything yourself, if someone submits a false DMCA notice you can file a counter notice with ease.
It is possible to be your agent and here is how to do that:
dmcaagentservice.com/12-steps-to-register-a-dmca-agent-with-the-copyright-office/
Please let me know if it does help your or if you have any other questions.