There is an issue where you cannot create a new keybinding on blizzard’s click casting ui for the hero talent “Chrono Flames”. If you try it refuses to save it.
I found a temporary work around where you can do so in a spec without that talent, as in bind “Living Flames” and it will update it when you switch to spec that has the “Chrono Flames” hero talent so you can click to cast it.
Good luck !