Sorry, but I don't understand where you are stuck. As you say, when using ESP-Now the master device has to have the MAC address of each slave as part of its code, unless, of course, you broadcast to all slaves listening by using a MAC address of FF:FF:FF:FF:FF:FF
Each slave ESP board has a MAC address which you can read and use in the master coder. How do you envisage using WiFi Manager to enter MAC addresses ?