Change the setting for "DNS Server" to manual or whatever the option is (you don't want Obtain DNS Servers Automatically).
Then enter the static IP of your Pi-Hole in the first set of blocks. Sometimes (not with all routers), the software requires an entry in the second set of boxes. If that is your case, enter the static IP of the Pi-Hole in those boxes as well.
Save changes, restart the router, renew the DHCP lease on your devices (this will give them the new DNS address of the Pi-Hole). You may have to clear DNS caches on connected clients, depending on the clients).