Why do you want to do this if you’re already using pihole as your primary DNS server?
Personally with your setup i would just configure the DNS forwarder in pfsense and send it over to pihole. OR get rid of pihole and use PfBlocker, if you really want to insist on using your firewall as a DNS resolver.
You could also (and this just seems needlessly complex to me) configure the upstream DNS server in pihole to be your pfsense box.
If it’s a windows environment the you MUST use DNS from you windows server. But you can forward your queries from windows DNS to pfsense real easy. I agree with @Stiehl that you should migrate to pfblockerng. This guide should help.
As suggested, you can configure pfSense to forward DNS queries to Pi-hole. This setup simplifies your DNS infrastructure by keeping Pi-hole as the primary DNS server for filtering and caching. If your current setup with Windows Server forwarding to Pi-hole is working well and meeting your needs, there may not be a compelling reason to introduce pfSense DNS Resolver into the mix. It’s essential to consider whether the added complexity of managing multiple DNS servers outweighs any potential benefits. When making changes to your DNS infrastructure, consider factors such as security, redundancy, and performance. Ensure that your chosen setup meets your organization’s requirements and aligns with best practices for DNS management.