How do I assign an action to long pressing the home button? I want to fix it so the home button long press kills a particular app. I can't see the option for home long press in Tasker or Macrodroid.

Galaxy S5, rooted, CM Marshmallow, Xposed installed.
Macrodroid doesn't offer an action based on  home key trigger. Macrodroid developer in app forums says


  It's impossible to create such a trigger because the home button is a fundamental part of the OS and there is no way to intercept the button press.


I don't know  Tasker, but guess the same limitations would apply, so you would need to find an alternative way. One method below:

Since you are on CM, you can use Gravity Box Xposed module

To kill foreground app

Navigation keys action &rarr; Home key &rarr; long press action &rarr; Kill foreground application 

To kill a particular app - combination of Gravity Box and Macrodroid

Navigation keys action &rarr; Home key &rarr; long press action &rarr; Custom actions &rarr; toggle applications to reveal shortcuts &rarr; Macrodroid Shortcuts &rarr; Add new macro

Trigger &rarr; Empty Trigger

Action : Kill application &rarr; choose application from drop down menu