after searching a loooooot, and after making numerous tests, I finally manage to use a regular TRS external mic on my FP2 (lineage OS 16 installed).
First, the FP2 audio plug is a TRRS (Tip-Ring-Ring-Slave, in other words it works with minijacks that have 3 black plastic rings) plug (like every mobile phone has).
So you will need an adapter TRS to TRRS (very easy to find).
But it is not sufficient. There is a trick to do in order to make the FP2 recognize your external mic as a mic: plug first the adapter only, the headphones icon should show up in the notification bar on the PF2, then it will disappear, and then the headset icon should finally appear. Then you can connect you mic to the adapter. In my case it works.
If you connect the adapter to the phone with the mic plugged into it already, the FP will recognize it as headphones, and it will not get the sound from the mic.