There are a few ways to add an external microphone, such as using a bluetooth microphone or plugging a mic into the headphone jack (where present).
But we recommend using a digital microphone. IK Multimedia sells a few, such as the:
IK Multimedia iRig Mic HD 2 (or the IK Multimedia iRig Mic Studio):
To plug this into an iPad with a lightning connector AND also keep the iPad charged, you need to buy these cables and connectors:
1. Connect the "Lightning to USB 3 Camera Adapter" to the Lightning port on the iPad
Note: Avoid non-Apple branded cables, we have found that they usually don't deliver the Fast Charge needed.
3. Plug the other end of the cable in #2 to "29W USB‑C Power Adapter
". This provides Fast Charging to keep the iPad charged even when it is also powering a Microphone (or ethernet adapter):
4. Plug a digital Microphone (or ethernet adapter) into the USB-A side of the Lightning to USB 3 Camera Adapter mentioned in #1.
NOTE: that if you are using a USB-C with a 2018 iPad, then you would NOT use these adapters. Rather you should look at a USB-C hub.