How do I use Xbox version of L4D2 with Keyboard and mouse?

You can't. It's a backwards compatible Xbox 360 game. You must use an Xbox controller.

There are some games that do use a keyboard/mouse combination if the player chooses to use it. https://www.windowscentral.com/list-xbox-one-mouse-and-keyboard-games

And here's some information on how to set up the Keyboard/Mouse on the console https://beta.support.xbox.com/help/Hardware-Network/accessories/mouse-keyboard

How do I start the game while using mouse and keyboard, I can’t start it with mouse and keyboard so I have to use the controller

I start up fortnite then plug in my keyboard and mouse and only my keyboard works how do i fix this ?

What model of keyboard and mouse are you using?. There is of course a work in progress for both, I highly suggest using a wired Keyboard and mouse only at this current moment on fortnite.

Both are wired but mouse wont work on any game is my mouse broke???

I start up fortnite then plug in my keyboard and mouse and only my keyboard works how do i fix this ?

What model of keyboard and mouse are you using?. There is of course a work in progress for both, I highly suggest using a wired Keyboard and mouse only at this current moment on fortnite.