How to map a custom jetbrains://xyz end point to an internal web server?

I have tried to ask this question but got no response to the question asked.
(see https://intellij-support.jetbrains.com/hc/en-us/community/posts/5001474093842-How-to-set-up-a-custom-jetbrains-xyz-end-point-in-a-plugin-?input_string=How%20to%20map%20a%20custom%20jetbrains%3A%2F%2Fxyz%20end%20point%20to%20an%20internal%20web%20server%3F)

How to map a custom jetbrains://xyz end point to an internal web server?
I.e., how to define in a plugin jetbrains:// route and a handler for that route.

(This is not a question how to start or use internal web server.)