The following is capturing instructions on how to add custom naming for an Arduino Leonardo (or generic pro-micro) for a DIY space mouse project.
Being new to Arduino IDE I found benefit in learning about several priciples from this video shared by the original poster.
Reference was taken from the following YouTube video (https://youtu.be/hoCOq9Ngp44?t=1156) approx 20min. Code section esp helpful.
Find the folder where the Arduino Hardware 'boards.txt' lives. Likely here:
'C:\Users\AppData\Local\Arduino15\packages\arduino\hardware\avr\1.8.7\boards.txt'