Download Kernel (linux-5.15.86) from custom Git repository

For my project, need to download the kernel from custom git repository (for example: https://custom/kernel.git, branch 5.15.86) , this rep is equal to kernel linux-5.15.86, that downloaded bydefault
I start from "make menuconfig" in /openwrt.
-> "Enter git repository to clone" - I print URL to my git rep.
-> "Enter path to local reference repository" - I print name of the branch
What is "Use external kernel tree " and how to use this option, maybe someone has already set up downloading the kernel from a custom repository?