Open up the console.
If you want the L key to be your key, you can paste this into your console.
alias "L Key"; bind L "L Key"; cl_scoreboard_mouse_enable_binding "L Key"
The alias, is the name that will be shown on the scoreboard as text, this can be anything.
alias "Nothing" bind L "Nothing" cl_scoreboard_mouse_enable_binding "Nothing"
This will show as "Nothing Enables Cursor" on your scoreboard.
Save it to your config, by executing the following command in your console.
And this is how mine looks like :D: