How to use an external microphone for input to via iPhone to hearing aid

I need to better select which person to listen to in a group- or table setting by using an external directional microphone connected to the Lightning port in my iPhone (7Plus). Is there a setting on the iPhone where I can tell the iPhone to choose the input from the Lightning port and not the internal microphone?

Are there any requirements to the output from the external microphone for iPhone to use the signal? Does it e.g. have to have a preamplifier?